## PHYSICAL NATURE OF MATTER

Physical Nature of Matter

→ Matter is made up of particles. All matter constitute of very small particles. These small particles are called atoms.

→ These particles of matter are too small so they cannot be seen by naked eyes or simple microscope.

→ Particles of matter are continuously moving as they posses kinetic energy, with the increase in temperature the kinetic energy of particles also increases so particle moves faster.

## characteristics - shape, volume, density

Learning Objective

To describe the solid, liquid and gas phases.

Water can take many forms. At low temperatures (below 0oC), it is a solid. When at "normal" temperatures (between 0oC and 100oC), it is a liquid. While at temperatures above 100oC, water is a gas (steam). The state that water is in depends upon the temperature. Each state has its own unique set of physical properties. Matter typically exists in one of three states: solid, liquid, or gas.

## 1. Matter and its Classification

Diffusion

Brownian Motion: The zig-zag or random path travelled by the particles of matter is called Brownian motion.

→ Intermixing of particles of two different types of matter on their own is called diffusion.

The rate of diffusion increases on increasing the temperature of the diffusing substance (by heating).

Example - The colour of ink spreading in water due to diffusion of particles of water.

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## 1. Matter and its Classification

Chapter 1

Matter in our surroundings

Introduction

• Everything in this universe is made of materials which scientist has names ‘matter’.
• The matter is made up of very small tiny particles. It is not continuous but is particulate.
• The matter is anything that occupies space and has mass.
• Particles of matter have space between them and are continuously moving.
• Particles of matter attract each other.

PHYSICAL NATURE OF MATTER

1. Matter is made up of particles

If we take a 100 mL beaker, filling half of it with water and dissolve some salt/sugar. We will observe that there is no rise in water level and the salt/sugar has spread throughout the water (shown in the fig).

1. How small are these particles of matter?
• The particles of matter are very small – they are small beyond our imagination.

If we dissolve few crystals of potassium permanganate in about 1000mL of water, we will see the colour has changed. It shows that there must be millions of tiny particles in just one crystal of potassium permanganate, which keep on dividing themselves into smaller and smaller particles.

## 1. Matter and its Classification

Chapter-1

Matter in Our Surroundings

### Introduction

Let us know about the meaning of the title of this chapter “matter in our surroundings”. So to know about it we need to understand the term ‘surrounding’ first. Do you know what is meant by the word ‘surrounding’? Surrounding can be anything that is around us like air, clouds, buildings, water bodies, etc. These all are surroundings. Do you know that all the things that surround us are matter? Anything around us that has some mass and occupies certain space when we keep it, is called matter. In this chapter, we will be studying matter, its properties and much more.

### Matter and its classification

Matter can be anything that occupies Space and has some Mass, (mass may be small or more doesn’t matter). Like we have LPG cylinder. Although it is filled with air only, but still it is too heavy. So, that is also matter as it has mass and occupies space. Even air is matter. If you see around, you will find that everything surrounding you is matter.

Classification of matter

Earlier when Matter was classified, then the ancient scientists said that matter was made up of five basic Elements called: Air, Fire, Earth, and Water & Sky. These five components are called as “panchtatva”.

But with the advancement in the field of science, it was concluded that matter can be classified on the basis of two main properties that is physical and chemical and therefore there are two main categories of matter, as follows –

Classification of Matter

• Chemical Classification
• Physical Classification

### Physical and Chemical classification

Physical Classification

It is done on the basis of Physical Properties, i.e. the properties that we can see like rigidity, colour etc.

Chemical Classification

Is done on the basis of Chemical Composition of Matter that means what kind of particles are present, how they react with other, if they do not react then why so. Let’s do some activities to study about Matter. These activities are conducted in order to know more about matter and learn about their properties.

Activity: Do you know how we come to know that matter is made up of small particles? Let’s perform an activity: for this we need the following things

• beaker, common salt, spoon glass rod

1. Fill beaker 3/4th with water mark the level of water on beaker, then add common salt to water.

2. On adding, we see that when we add salt to water it settles down.

3. On stirring, it starts disappearing and then a stage comes when it disappears completely.

Let s see what does this observation proves

• As we are not able to see salt particles, this shows that it is not one continuous state of matter, instead it is formed of small particles.
• Actually, on stirring, the salt particles get dissolved in water but still the level of water remains same. This shows that there are spaces between the particles of water which are occupied by the small salt particles.

Another activity: If we consider about the motion of particles. Do you actually think that they can show movement? If yes, lets track their movement?

In order to perform the activity, the following things are required – Stand, Agarbatti Match box

1. Now let’s take one stick out of packet, hold it on stand, place it in a corner of the room and light it. We will see that its fragrance gets spread evenly in the room.

2. The fragrance can be felt in whole room. It is not confined to one corner. So the observation is that “The whole room starts smelling very good“

The Conclusion to an activity

• Is that particles of matter are not stationary. They are moving  continuously and when they drift through air, we can smell the pleasant fragrance.
• The reason behind it is that when we light the stick, the heat energy supplied is taken by the particles, which increases their kinetic energy and this makes the fragrant particles to move rapidly. They easily drift through the air and spread fragrance.

You will be surprised to know the actual phenomenon involved behind this activity- The Phenomenon is called Diffusion.

Diffusion

It is the movement of any substance from a place of higher concentration to a place of lower concentration, or It can also be defined as the intermixing of different substances.

Activity to show diffusion: let’s take two flasks connected to each other by a knob.

Now let’s say on one side there is gas A and on the other side, there is gas B. When knob is closed, both the gases are in their own flasks. But when the knob is opened the two gases intermix rapidl y. As a result, both the flasks will have A and B gas particles. This intermixing is called diffusion. It is best shown by gases as the gas particles have maximum kinetic energy.

Activity to prove: particles of matter attract each other

Let’s take different substances like an iron nail, piece of wood, rubber band

1. Now try to break them by using hammer.

2. As we know, all three belong to same states – solids.

3. Can you guess what we noticed?

4. We noticed that it is easier to hammer down rubber band as compared to wood and iron nail.

5. The reason is that the particles in iron nail are so closely packed that it is difficult to separate those using hammer in comparison to wood or rubber band.

Conclusion of this activity:

In iron, the force of attraction between particles is maximum as particles are closer whereas in others they are not so close.

All these activities lead to the formulation of more information about matter and its properties which lead to the introduction of the Theory of Molecular Structure of Matter.

## 2. Molecular Theory of Matter

Characteristics of Particles of Matter

• Particles of matter have space between them

→ Gas can be compressed a lot because of the space between their particles.

Activity:- take some water in a beaker and note its level. Dissolve some salt or sugar in it with the help of  a glass rod. The salt dissolves in the water but the level of water dose not change. This is because the particles of salt get the space between the particles of water.

→ Additionally you would notice that there is no rise of water level takes place when one or two teaspoon of sugar/salt is added into a glass of water, this is because sugar/salt particles get adjusted in the space between the particles of water and no rise in the water level comes in result.

• Particles of matter attract each other because of force of attraction.

Activity:- Take an iron nail, a piece of chalk and rubber band. Try breaking them hummering, cutting or stretching. It is more easier to break the chalk, less easier to break the rubber band and difficult to break the iron nail. this is because the particles in the iron nail are held together with greater force than in the rubber band or challk.

→ Force of attraction between particles of matter keeps the particles bonded together. Therefore attraction between particles of solid is greatest, between particles of liquid is moderate and between particles of gas is lowest.

→ Because of the lowest force of attraction between the particles of gas we can move our hand through

air easily. To move our hand in liquid, such as water, we have to apply some force, but a solid such as wood, we cannot move our hand.

→This is because the force of attraction between particles of gas is almost negligible, in liquid the forces of attraction is moderate but it is greatest in solid.

→ The force of attraction between particles of solid, liquid and gas can be arranged in decreases in order as follows:

Solid > Liquid > Gas

• Particles of matter are continuously moving –

Activity :- Take some water in a beaker and put a drop of blue or red ink slowly along the sides of the beaker. Leave it undisturbed for a few hours. The ink spreads evenly throughout the water due to the movement of the particles of water and ink.

The intermixing of two or more different types of matter on their own is called diffusion.

• Matter is made up of small particles –

Activity :- Dissolve 2 —3 crystals of potassium permanganate in I OOml of water in a beaker. Take lOmI of this solution and dissolve in lOOmI of water. Take lOmI of this solution and dissolve In lOOmI of water. Repeat this process 5— 6 times. This shows that a few crystals of potassium permanganate can colour a large volume of water because there are millions of tiny particles in each crystal

Classification of states of matter

States of Matter

• Solid State

→ The space between the particles is very less.

→ The force of attraction between the particles is strong. Thus, particles in a solid are closely packed.

→ Solids maintain their shape even when they are subjected to external force i.e. they are rigid

→ Solids cannot be compressed.

→ The kinetic energy of the particles is very less and so solids have an orderly arrangement of particles. Therefore, solids have a fixed shape and volume.

• Liquid State

→ The space between the particles is slightly more as compared to solids, but still very less as compared to gases. The particles of a liquid can slip and slide over each other.

→ The force of attraction between the particles is strong enough to hold the particles together not strong enough to hold the particles in a fixed position.

→ Liquids do not have a fixed shape but have a fixed volume. Liquids take up the shape of the container in which they are poured.

→ The kinetic energy of the particles is more than that of solids. Thus, liquids have a disorderly arrangement of particles compared solids.

→ Liquids cannot be compressed much. The compressibility of liquids is almost negligible.

• Gaseous State

→ The particles are much farther apart from one another as compared to solids and liquids.They have a very disorderly arrangement of particles compared to the solids and liquids.

→ The force of attraction between the particles is negligible, hence particles of a gas move free in all the directions.Gases thus can mix or diffuse into other gases.

→ The particles of a gas have maximum kinetic energy. They move with high speed in all directions and can exert pressure on the walls of its container.

→ Gases neither have a definite shape nor a definite volume.They fill up the container completely

→ Gases can be compressed easily. Example: the LPG cylinders used at home and the CNG cylinder used in vehicles.

## 2. Molecular Theory of Matter

CHARACTERISTICS OF PARTICLES OF MATTER

1. Particles of matter have space between them

2.Particles of matter are co ntinuously moving –

• They possess kinetic energy.
• Increase in temperature also increases the kinetic energy of the particles
• Thus, Particles of matter intermix on their own with each other.
• They do so by getting into the spaces between the particles.
• This intermixing of particles of two different types of matter on their own is called diffusion
• On heating, diffusion becomes faster.

3. Particles of matter attract each other.

• The strength of this force of attraction varies from one kind of matter to another.

## 2. Molecular Theory of Matter

### Molecular Theory of Matter

Theory of molecular structure of matter

According to this theory,

• Matter is made up of small particles called molecules.
• They are in a state of continuous motion. Due to this, we can say that they possess K.E.
• K.E. Increases with ↑ se in temperature.
• K.E. Is maximum in gases and least in solids.
• The space between molecules is called intermolecular space which is found to be least in solids and maximum in gases.
• There is a force that exists between particles of matter and is called intermolecular force
• Intermolecular force ↓ ses with increase in Intermolecular space

Terminology involved

Before we start with more properties of matter let us know some terms which will be used in this chapter.

• Matter: Anything that occupies space is matter.
• Material: The term is used to describe a particular kind of matter.

Materials are of two Types:

• Homogenous
• Heterogeneous

Homogeneous Materials: Are those that have the same composition and same properties throughout the sample. For example, if you take water in a glass and add salt to it and stir it. It will result in formation of a mixture that has same properties and uniform composition (that is no distinct layers are seen). They mix thoroughly. If you take sample from any part of that mixture, it will show similar properties.

Heterogeneous Materials: are those that have different composition and different properties in different parts of the sample. For example, if we take sand in water, then we will see that inspite of stirring, it does not dissolve in water. It will settle at the bottom of the container and few particles will be seen floating in the water. If you take a sample from any part of it, it will show different properties.

Molecule: Is a term used for particular type of matter that has independent existence in nature like oxygen molecule, carbon dioxide molecule, water molecule, etc.

Molecular structures and properties of solids, liquids and gases

Now let us do Physical Classification of Matter: this classification is done on the basis of physical properties of matter, that is the properties that can be seen or felt by touching, looking etc.

According to it, matter is divided into 3 types:

• Solids
• Liquids
• Gases

Solids

if you take any solid let’s say we consider wood, what properties can you find in it by looking at it.

• First property that we can make out is: That it has fixed shape and volume: we realize this because when wood is kept on the floor or any surface, it occupies definite space.
• Another property that we notice is: It is not compressible: if we it try to squeeze it or change its shape, we cannot do so. This is probably due to the reason that there is no space between solid particles and in order to compress it, there should be space between particles. This is because when we apply force, the particles fill those empty spaces and come closer.
• Another property: They don’t need container to hold them: we don’t have to put an almirah or any other solid in a container. it can be kept as such on the surface.
• They do not flow: This is because when wood is kept at a place, it remains at the same place. It doesn’t flow or move on its own.
• Their diffusion tendency is nil: it is seen that wood doesn’t move from one side of room to another side of room on its own as the particles do not have that much energy that they keep on moving.

Let us try to understand solids on the basis of theory of molecular structure:

Explanation of Solids on the basis of Molecular Structure

If we see any solid we observe that in solids, particles are tightly packed. They are very closely packed due to which there is hardly any space between particles or we can say that intermolecular spaces are very less and when particles are closer, that means the intermolecular forces of attraction are stronger. As a result, solids have fixed shape, volume and can’t be compressed.

Now do you understand that why wood or any other solid doesn’t flow or compress or diffuse and why they possess high density.

Liquids

Let’s sum up the properties of gases in comparison to solids and liquids

• They have neither fixed shape nor fixed volume
• They are highly compressible
• They can flow, diffuse to great extent
• They have very very low density
• They can fill the entire space

Explanation of gases on the basis of molecular structure

In gases the particles are very far apart. Due to this, they have high kinetic energy and keep on moving randomly. As a result, the intermolecular spaces are very large and intermolecular forces are almost nil.

## 3. Plasma and BEC State, Temperature

Change of State of Matter

→ The phenomenon of change from one state of matter to another, and then back to the original state is called the interconversion of states of matter.

→ Matter Can Change its State. Water can exist in three states of matter:

• Solid as ice

• Liquid as water

• Gas as water vapour

Effect of Temperature

→ On increasing the temperature of solids, the kinetic energy of the particles increases which overcomes the forces of attraction between the particles thereby solid melts and is converted into liquid.

→ The temperature at which a solid melts to become a liquid at the atmospheric pressure is called its

melting point.

The melting point of ice is 273.16 K.

The process of melting, that is, change of solid state into liquid state is also known as fusion.

## 3. Plasma and BEC State, Temperature

STATES OF MATTER

Matter around us exists in three different states– solid, liquid and gas, dependent on the characteristics of the particles of matter.

1. SOLID STATE – Solids have a definite shape, distinct boundaries and fixed volumes, that is, have negligible compressibility.

• Solids may break under force but it is difficult to change their shape, so they are rigid.

Examples, a pen, a book, a needle and a piece of wooden stick, a granule of sugar.

• There are objects that are solid in state but seems do not follow the above rule but actually they do
• A rubber band changes shape under force and regains the same shape when the force is removed. If excessive force is applied, it breaks.
• A sponge has minute holes, in which air is trapped, when we press it, the air is expelled out and we are able to compress it.

2. LIQUID STATE – liquids have no fixed shape but have a fixed volume.

• They take up the shape of the container in which they are kept.
• Liquids flow and change shape, so they are not rigid but can be called fluid.
• Solids, liquids and gases can diffuse into liquids (e.g. oxygen and carbon dioxide dissolves in water, which helps the survival of aquatic animals and plants)
• The rate of diffusion of liquids is higher than that of solids.
• This is because in the liquid state, particles move freely and have greater space between each other as compared to particles in the solid state.

3. GASEOUS STATE – gases are highly compressible as compared to solids and liquids

• Due to its high compressibility, large volumes of a gas can be compressed into a small cylinder and transported easily
• Examples: liquefied petroleum gas (LPG) cylinder, Compressed natural gas (CNG) fuel tanks
• Due to high speed of particles and large space between them, gases show the property of diffusing very fast into other gases.
• Rate of diffusion is much faster than solids and liquids
• In the gaseous state, the particles move about randomly at high speed.
• Due to this random movement, they exert pressure which is the force exerted by each gas particles per unit area on the walls of the container.

## 3. Plasma and BEC State, Temperature

Plasma and Bose Einstein condensate state

Two states of matter are plasma and BEC, let’s understand them:

Plasma: It consists of ionized gas, such that its particles are super energetic and super excited.  In devices such as tube lights and CFL, the gases get ionized on the passage of current and glow with the color depending upon the nature of the gas. For Example: Neon gas emits red glow, argon emits green – yellow, etc.

Bose Einstein Condensate State: It is the fifth state of matter and it is obtained on super cooling the gas at almost absolute Kelvin zero temperature.

Temperature and temperature scales

You often fall sick, sometimes due to high fever. Then the first step that has to be taken is to measure temperature by using thermometer which records the temperature of the body in a  given scale.

Temperature is the degree of hotness or coldness of body or We can also say that temperature measures the extent of motion of atoms. It is measured with the help of thermometer.

There are different scales on which temperature can be recorded:

Commonly used scales are Celsius and Kelvin

Celsius Scale:

• It is written as oC
• On it the lower fixed point (at which ice melts) is O oC
• The upper fixed point on it (at which water boils) is 100 oC

Kelvin Scale:

• The lower fixed point is- 273K
• The upper fixed point is 373 K
• Kelvin scale is called as absolute zero scale of temperature because on it the lowest possible temperature is zero

Relation between K & oC :

These scales are interconvertable by using the well defined relations like 0 oC = 273 K

So, K = oC + 273           or oC = K -273

For example, if we need to convert 25 oC to K then, we need to add 273 to it and it comes out to be 298K.

## 4. Interconversion of States of Matter

• Latent heat

The hidden heat which breaks the force of attraction between the molecules is known as the latent heat. Since, the heat energy is hidden in the bulk of the matter, it is called latent heat.

• Latent heat of fusion

→ The heat energy required to convert 1 kilogram of a solid into liquid at atmospheric pressure,at melting point, is known as the latent heat of fusion.

→ The temperature at which a liquid starts boiling, at atmospheric pressure, is called its boiling point.

• Latent heat of vaporisation

→ The heat energy required to convert 1 kilogram of liquid into gas, at atmospheric pressure, at

boiling point, is known as the latent heat of vaporisation

→ The process, in which a gas, on cooling, turns into a liquid at a specific temperature is called

condensation or liquefaction.

Formation of clouds is due to the condensation of water vapour from the Earth’s surface.

→ The temperature at which the state of a substance changes from a liquid to a solid is called freezing point of that substance.

→ When a solid melts, its temperature remains the same because heat gets used up in changing the state by overcoming the forces of attraction between the particles. It is considered that it gets hidden into the contents of the beaker and is known as the latent heat.

→ Water vapour at 373 K have more energy than water at the same temperature because particles in steam have absorbed extra energy in the form of latent heat of vaporisation.

## 4. Interconversion of States of Matter

CAN MATTER CHANGE ITS STATE?

Yes, they do change under various circumstances and factors which we will study below

1. Effect of change of temperature

• On increasing the temperature of solids, the kinetic energy of the particles increases.
• Due to the increase in kinetic energy, the particles start vibrating with greater speed.
• The energy supplied by heat overcomes the forces of attraction between the particles.
• The particles leave their fixed positions and start moving more freely.
• A stage is reached when the solid melts and is converted to a liquid.
• The minimum temperature at which a solid melts to become a liquid at the atmospheric pressure is called its melting point.
• The melting point of ice is 0° C (273.15 K).
• The process of melting, is also known as fusion.
• When a solid melts, its temperature remains the same
• This increase in temperature (heat) of solids is used up in changing the state by overcoming the forces of attraction between the particles without showing any rise in temperature
• This hidden heat is called latent heat
• Therefore, the amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is known as the latent heat of fusion.
• So, particles in water at 0° C (273 K) have more energy as compared to particles in ice at the same temperature.
• Similarly, when we supply heat energy to water, particles start moving faster.
• At a certain temperature, a point is reached when the particles have enough energy to break free from the forces of attraction of each other.
• At this temperature the liquid starts changing into gas.
• The temperature at which a liquid starts boiling at the atmospheric pressure is known as its boiling point.
• Boiling is a bulk phenomenon i.e. each particles of the liquid gain enough energy to change into the vapour state.
• For water this temperature is 373 K (100°C = 273 + 100 = 373 K).
• The input energy required to change the state from liquid to vapor at constant temperature is called the latent heat of vaporization
• Water vapour at 373 K (100° C) have more energy than water at the same temperature. This is because particles in steam have absorbed extra energy in the form of latent heat of vaporization.

• A change of state directly from solid to gas without changing into liquid state is called sublimation. Example, vaporization of camphor
• The direct change of gas to solid without changing into liquid is called deposition. Example, soot in the chimney, making dry ice (solid carbon dioxide).

2. Effect of change of pressure

• Applying pressure and reducing temperature can liquefy gases. Examples, LPG, liquid nitrogen.

## 4. Interconversion of States of Matter

Interconversion of States of Matter

You must have noticed that if we keep ice out in kitchen, with time it gets converted into water or when water is heating and you forget to turn the gas knob off, then it keeps boiling and a stage is reached when the vessel gets empty and no water is seen inside. This is due to interconversion of states of matter.

The phenomenon due to which matter changes from one state to another and then back to its original state on altering the conditions of temperature and pressure is called interconversion of states of matter.

The state of the substance can also be changed by heating or by applying pressure.

As you all have LPG cylinders at home or must have seen CNG cylinders installed in cars, these cylinders contain Liquefied Petroleum Gas or Compressed Natural Gas which means that in these cylinders the gases are liquefied by applying pressure. So, keep in mind that on applying pressure the gas can change into liquid. Similarly, on applying pressure, liquid changes into solid.

For example, when pressure is applied on Carbon Dioxide gas, it forms dry ice (Solid state). So, interconversion tells us about the change of one state of matter in to another state.

Solid ↔ liquid ↔ gas

1. The conversion of solid to liquid is called melting and it occurs at fixed temperature called melting point

2. The conversion of liquid to gas is called evaporation and it occurs at fixed temperature called as boiling point

3. The conversion of gas to liquid is called condensation or liquefaction and it occurs at liquefaction point

4. The conversion of liquid to solid is called freezing and it occurs at temperature called freezing point.

5. The conversion of solid liquid gas involves absorption of heat energy

6. And the conversion of gas liquid solid involves release of heat energy

The energy which is required to change the state whether released or absorbed is called latent heat

Latent heat

It is the amount of heat released or absorbed during change of state of a substance.

Solid liquid

Like when we convert Solid to Liquid, heat is supplied and that heat is called latent heat of fusion that is –

Latent Heat of Fusion:

it is the amount of heat required to convert one unit mass of substance from solid to liquid state at its melting point. Example: for ice it is equal to 3.34 x 105 3/Kg

Change of state of solid to liquid on the basis of molecular structure

When we heat any solid substance, the particles gain energy and start vibrating with greater amplitude. Their kinetic energy increases due to which their intermolecular space increases and intermolecular force decreases and it gets converted into liquid state.

Liquid state gaseous state

Boiling or Vaporization: it is the change of liquid state of a substance to gaseous state at boiling point. Boiling Pt. => Is the temperature at which liquid changes to gaseous state on heating. Boiling point of water => 100 oC

Latent heat of Vaporization:

it is the amount of heat required to convert one unit mass of substance from liquid to vapour at its boiling point. For example, for H2O it is equal to 22.6 x 105 J/kg.

Change of state of liquid state to gaseous state on the basis of molecular structure

When we heat any liquid substance, the particles gain energy. As a result, their kinetic energy increases and they starts vibrating with greater amplitude due to which their intermolecular space increases and intermolecular force decreases and they get converted into gaseous state.

Gas Liquid

Condensation: It is the change of gaseous state to liquid state on cooling. This occurs at a temperature below the boiling point of substance

Change of state of gaseous state to liquid state on the basis of molecular structure

It is done by applying pressure and reducing temperature Explanation: When high pressure is applied on gas, distance between particles decreases & particles start attracting each other & gas gets converted in to liquid state.

Solid gas

You must have noticed that the naphthalene balls are used to prevent growth of insects in boxes of beds. With time, these balls disappear. Do you know what is the phenomenon behind this disappearance?

Sublimation: It is the change of solid state directly to gaseous state on heating & vapours back to solid on cooling.

It involves two terms, that is

1. Sublime: A gaseous state that is formed due to sublimation of solid

2. Sublimate: It is the solid that sublimes

Example of Sublimation: Sublimation of Naphthalene Balls

Evaporation

You must have seen that when wet clothes are spread under sun, they dry up or the wheat grains are often dried under the Sun before sending to mill for converting to flour. This is due to the phenomenon of evaporation that is taking place.

Evaporation is the process of change of liquid into vapor at temperature below its boiling point.

Explanation of Evaporation on the basis of Molecular Structure

During evaporation, the liquid particles are heated or they take the heat energy from the surroundings. When heat is taken by particles, the energy of particles increases due to which they start moving rapidly.

As a result, they collide with one another. In the course of collisions, some particles loose energy and some gain energy. As a result, particles with higher energy overcome their force of attraction and escape into the atmosphere.

Factors on which Evaporation Depends:

• Surface area
• Temperature
• Humidity
• Wind speed

Lets explain the dependence of the phenomenon on these given factors:

1. Surface Area – More is the exposed surface area of any substance more liquid droplets will change to vapor state For xample: Drying of Wet Clothes We have seen that clothes dry faster when they are spread on a line. The reason is that when we spread them, the surface area gets increased, therefore, evaporation gets increased as more and more molecules are exposed.

2. Temperature: Increase in temperature, increases the Kinetic Energy of molecules. They break their force of attraction, hence, escape or evaporate readily. For example: Formation of Water vapor on cup of Hot Tea.It is because as temperature rises, the particles start moving randomly, therefore, evaporation rate is increased. Therefore, when they escape, they also collide with the cup and get condensed.

3. Humidity: More is Humidity less in the Evaporation For example- Clothes dry with difficulty on a humid day: because as humidity is more, that is more vapour in the atmosphere. Due to which the escaping tendency of vapours decreases so as the rate of evaporation decreases.

4. Wind Speed: With increase in speed of wind, evaporation increases. That is the reason why clothes dry faster if there is a windy day. As the speed of wind is high, the liquid particles take up the energy. As a result, their kinetic energy increases and they escape more readily and more easily.

Evaporation Cause Cooling Effect

Evaporation is a surface phenomenon. The particles at the surface having high K.E. break up their force of attraction & form vapours. Since the energy is lost, that means temperature gets lowered and low temperature means cooling effect so, evaporation results in cooling effect

Q. Do you know why we feel more comfortable if we wear cotton clothes in summers?

A. It is because the cotton clothes are very good absorbers of Water. They rapidly absorb sweat and then evaporate taking large amount of heat from our body thereby, giving a cool sensation.

## 5. Evaporation

Sublimation –

The change of state of a substance directly from a solid to gas, without changing into the liquid

state (or vice versa) is called sublimation.

Examples – Naphthalene balls, Camphor, Ammonium chloride, iodine etc.

## 5. Evaporation

EVAPORATION

• In liquids, a small fraction of particles at the surface, having higher kinetic energy, is able to break away from the forces of attraction of other particles and gets converted into vapour. This phenomenon of change of a liquid into vapours at any temperature below its boiling point is called evaporation.

1.Factors affecting evaporation

• The rate of evaporation increases with
• increase of surface area
• increase of temperature
• decrease in humidity
• increase in wind speed

1. How does evaporation cause cooling?

The particles of liquid absorb energy from the surrounding to regain the energy lost during evaporation. This absorption of energy from the surroundings makes the surroundings cold.

-Why should we wear cotton clothes in summer?

* During summer, we perspire more because of the mechanism of our body which keeps us cool. We know that during evaporation, the particles at the surface of the liquid gain energy from the surroundings or body surface and change into vapour. The heat energy equal to the latent heat of vaporisation is absorbed from the body leaving the body cool. Cotton, being a good absorber of water helps in absorbing the sweat and exposing it to the atmosphere for easy evaporation.

-Why do we see water droplets on the outer surface of a glass containing ice-cold water?

* If we take some ice-cold water in a tumbler. Soon water droplets forms on the outer surface of the tumbler. The water vapour present in air, on coming in contact with the cold glass of water, loses energy and gets converted to liquid state, which we see as water droplets.

## EFFECT OF CHANGE OF PRESSURE

Effect of change of pressure

→ Gases can be liquefied by applying pressure and reducing the temperature. When a high pressure is applied to a gas, it gets compressed and if the temperature is lowered, the gas is liquefied.

→ Solid CO2 gets converted directly to gaseous state on decrease of pressure to 1 atmosphere without coming into liquid state. This is the reason that solid carbon dioxide is also known as dry ice

## EVAPORATION

Evaporation

→ The process of conversion of a substance from the liquid state to the gaseous state at any temperature below its boiling point is called evaporation or vaporisation.

→ Evaporation is a surface phenomenon.

• Factors affecting the rate of evaporation

Surface Area The rate of evaporation increases on increasing the surface area of the liquid.

Temperature -  The rate of evaporation increases with an increase in temperature.

Humidity - Decrease in the humidity increases the rate of evaporation.

Wind Speed - An increase in the wind speed increases the rate of evaporation.

• Evaporation causes cooling

→ The particles of liquid absorb energy from the surrounding to regain the energy lost during

evaporation. This absorption of energy from the surroundings make the surroundings cold.

→ Lately, scientists are talking about five states of matter or five phases of matter. These are-so

liquids, gases, plasmas and the Bose–Einstein condensate.

## OTHERS STATES OF MATTER

Plasma

The state consists of super energetic and super excited particles. These particles are in the form

ionised gases. The fluorescent tube and neon sign bulbs consist of plasma.

Bose-Einstein Condensate

→ Indian physicist Satyendra Nath Bose made a study regarding the fifth state of matter. Base

his study, Albert Einstein predicted a fifth state of matter called the Bose-Einstein Condensate.

→ The Bose-Einstein Condensate or BEC is formed by cooling a gas of extremely low density to

low temperatures.

Conversion of Temperature

→ Kelvin is the SI unit of temperature, C = 273.16

For convenience, we take 0° C = 273 K

after rounding off the decimal. To change a temperature

on the Kelvin scale to the Celsius scale you have to subtract 273 from the given temperature, and to convert a temperature on the

Celsius scale to the Kelvin scale you have to add 273 to the given temperature.

Atmosphere (atm) is a unit of measuring pressure exerted by a gas. The unit of pressure is Pascal.

(Pa): 1 atmosphere = 101325 Pa.

The pressure of air in atmosphere is called atmospheric pressure & atmospheric pressure at sea level is 1 atmosphere, and is taken as the normal atmospheric

pressure.

Some measurable quantities and its units

## 1. Pure Substance

Introduction

Anything which occupies space and has mass is called matter.

Matter can be divided in two categories.

(i) Pure Substance: It consists of single types of particles which are same in their chemical nature.

(ii) Mixtures: Mixture consists of two or more particles.

Mixture and its types

Mixture consists of more than one kind of pure substances which can be separated by physical method.

Mixtures are of two types

(i) Homogeneous mixture

(ii) Heterogeneous mixture

(i) Homogeneous mixture: A mixture is said to be homogeneous if all the components of the

mixture are uniformly mixed and there are no boundaries of separation between them.

Ex: Sugar in water, etc.

(ii) Heterogeneous mixtures: A mixture is said to be heterogeneous if all the components of the

mixture are not uniformly mixed and there are visible boundaries of separation between them

Ex: Water and sand, Air etc.

## 1. Pure Substance

Is Matter around us pure

Introduction

When we talk about pure, it means that all the constituent particles of that substance are the same in their chemical nature. A pure substance consists of a single type of a particles. What is the type of pure substances?

• Elements

• Robert Boyle A was the first scientist to use the term element in 1661.
• Antoine Laurent Lavoisier (1743–94), a French chemist, was the first to establish an experimentally useful definition of an element.
• Elements can be normally divided into metals, non – metals and metalloids.

Metals

• Metals usually show some or all of the following properties:
• They have a lustre (shine).
• They conduct heat and electricity.
• They are ductile (can be drawn into wires).
• They are malleable (can be hammered into thin sheets).
• They are sonorous (make a ringing sound when hit).

# Examples of metals are gold, silver, copper, iron, sodium, potassium etc.

# Mercury is the only metal that is liquid at room temperature.

Non metals

• Non – metals usually show some or all of the following properties:
• They are poor conductors of heat and electricity.
• They are not lustrous, sonorous or malleable.

# Examples of non – metals are hydrogen.  oxygen, iodine, carbon (coal, coke).  bromine, chlorine etc.

Metalloids

• Metallaoids have intermediate properties between of metals and non – metals.

# Examples are boron, silicon, germanium etc.

Mixture and compound

 Mixture Compound 1. Elements or compounds are simple calling so new substance is formed. Compound 1. Substances Are Reated Together with each other to make a new substance. 2. Elements do not combine in a fixed ratio. 2. Compositions the the component is Fixed i.e. , They combine together in a fixed ratio according to their masses. 3. A mixture shows the properties of its components 3. compound does not show the Properties of component elements. 4. Components can be easily separated by any mechanical method which is suitable. 4. components can not be separated from each other by simple mechanical methods.

## 1. Pure Substance

Chapter-2

Is Matter around us Pure

### Is Matter around us Pure Introduction

We come across different substances like water that we drink; salt that we add in food for taste, milk that we drink as it provides us with calcium and other minerals, soaps that we use to wash clothes, paint that we use to colour walls and so on. These substances have different nature, different properties. They can be pure or there can be some impurities in them. Let us see what do we understand by the words pure or impure substance[12]

We come across different substances like water that we drink; salt that we add in food for taste, milk that we drink as it provides us with calcium and other minerals, soaps that we use to wash clothes, paint that we use to colour walls and so on. These substances have different nature, different properties. They can be pure or there can be some impurities in them. Let us see what do we understand by the words pure or impure substances[15]

We come across different substances like water that we drink; salt that we add in food for taste, milk that we drink as it provides us with calcium and other minerals, soaps that we use to wash clothes, paint that we use to colour walls and so on. These substances have different nature, different properties. They can be pure or there can be some impurities in them. Let us see what do we understand by the words pure or impure substances[20]

### Pure substance

They are substances that are made up of a single type of particle. But there are certain characteristics that determine the purity of a substance. Let us see what all properties they possess.

Characteristics of a pure substance are as follows

1. They all have a uniform composition (homogeneous).

2. They can not be separated into their constituents.

3. They have fixed melting and boiling points.

4. They always have the same characteristic properties.

Two categories of substances fall under pure substance. They are as follows

1. Elements

2. Compounds

Element

As we know, an atom is the smallest particle that may or may not exist independently. These atoms unite together and form an element. This element can exist independently but it can not be broken into atoms as they are not visible. About 118 elements are known so far and still many discoveries are in the pipeline. Elements can be prepared artificially in lab, also by transmutation process. The elements having atomic number more than 92 are manmade and are called transuranic elements. An element is defined as a substance that can not be broken into simpler substances as it is formed of atoms and atoms can not be seen.

The characteristics of elements are as follows

1. They are made up of atoms

2. The physical & chemical properties of an element are due to the arrangement of atoms.

3. They can occur in nature in free or combined form.

4. They can be prepared artificially by nuclear reaction.

5. They can be solids, liquids (only 3) or gases (11) at normal room temperature.

Metals

In daily routine, we use a lot of substances that fall in this category like the vehicles which are usually made up of iron, furniture, utensils and many more. Let us see their properties. Those substances which have a tendency to lose electrons and form positively charged species, that is cation are called metals.

Na – 1e- → Na+ (cation)

2, 8, 1 2, 8

Physical properties of metals are as follows

• They are malleable like Al(aluminum), Ag(silver) [ except alkali metals that is Na,K,Li etc]
• They are ductile like Al, Ag, etc.
• They are lustrous (except – Hg(mercury), Cs(cesium), Ga (gallium)) .
• They are hard (except Na & K ).
• They are good conductors of heat & electricity.
• They have high melting and boiling points [except Na, k, Ca they have low boiling and melting point] .
• They are Sonorous (that is when hit, they produce a sound).

Chemical Properties of metals are as follows

1. Reaction with oxygen

Metals react with oxygen to form oxides.

M + O2 → Metal oxides

All Metal oxides are basic in nature i.e. they turn red litmus blue but some metal oxides are amphoteric in nature (that is, they have acidic & basic nature) like Al2O3 (aluminum oxide) & ZnO (zinc oxide).

2. Reaction with dilute acids

Metals react with dilute acids to form a salt and H2 gas.

M + HX → MX + H2

Example: Na + HCl → NaCl + H2

The hydrogen gas, when comes in contact with air, burns with a popping sound.

Non-Metals

Non – Metals are those elements that always gain electrons and form anions (negatively – charged ions).

Example: Cl + e- → Cl- anion

2, 8, 7 2, 8, 8

Physical properties of non – metals are as follows

• They are liquid or gases (only one exists in liquid form – that is Bromine) except C, S, P, I that are solids.
• They are non – malleable and non – ductile because to make wires or sheets, we need to hammer them, but as they are brittle, they break.
• They are bad conductors of heat and electricity (like graphite).
• They have low melting and boiling point (except B, C which have high melting and boiling points).
• They are non sonorous.
• They are non lustrous (except graphite and iodine).

Chemical properties of non – metals are as follows

1. Reaction with oxygen

Non – metals react with O2 to form Non Metal oxide with respective formula as given below:

N + O2 → non metal oxide

They are acidic in nature and turn blue litmus red.

2. Reaction with dilute acids

Non metals do not react with die acids as they do not have sufficient electrons.

Metalloids

They are those which have properties similar to metals and non-metals. Few elements exist as Noble gases. They are those which are stable elements as they have a stable configuration and generally exist free in nature as they do not need to combine with other elements because they have a stable electronic configuration. The noble gases are He (helium), Ne (neon), Ar (Argon), Kr (krypton), Xe (xenon) and radon.

Compounds

We use so many compounds like salt, water, fertilizers, etc. Let us learn about their properties. They are formed when 2 or more elements combine in a fixed whole number ratio.  Example: H2O (water), NaCl (sodium chloride), etc.

Characteristics of compounds are as follows

• They are homogenous.
• The properties of a compound are entirely different from its constituents.
• The constituents can not be separated by physical methods.
• They have fixed properties like Melting point and Boiling point.
• The formation of compound is accompanied by energy changes.

## 2. Impure Substance

Solution and its properties

A solution is a homogeneous mixture of two or more substances. Ex: Lemonade, soda water etc

A solution has two components:

(i) Solvent

(ii) Solute

(i) Solvent: The component of the solution that dissolves the other component in it (usually the component present in larger amount) is called the solvent.

(ii) Solute: The component of the solution that is dissolved in the solvent (usually present in less quantity) is called the solute.

Properties of Solution:

1. A solution is a homogeneous mixture.

2. The particles of a solution are smaller than 1 nm

(10-9 ) in diameter which cannot be seen by naked

eyes.

3. They do not scatter a beam of light passing through the solution that is they don’t show tyndall effect. So, the path of light is not visible in a solution.

4. The solute particles cannot be separated from the mixture by the process of filtration.

5. The solution is stable and solute particles do not settle down when left undisturbed.

Concentration of a solution

(i) Saturated solution: When no more amount of solute can be dissolved in a solution at a give temperature, it is called a saturated solution.

(ii) Unsaturated solution: When more amount of solute can be dissolved in a solution at a give temperature, it is called a saturated solution.

(ii) Solubility: The amount of the solute present in the saturated solution at the given temperature

called its solubility.

The concentration of a solution is the amount of solute present in a given amount (mass or volume) of solution. Also, the amount of solute dissolved in a given mass or volume of solvent is called concentration of solution.

Concentration of solution = Amount of solute/Amount of solvent or Amount of solute/Amount in

solution (Here, amount means mass or volume).

Two methods of finding concentration of solution:

(i) Mass by mass percentage of a solution = (Mass of solute/Mass of solution) ×100

(ii) Mass by volume percentage of a solution = (Mass of solute/Volume of solution) ×100

## 2. Impure Substance

But when we see around us, we observe most of the matter around us exists as mixtures of two or more pure components. For example: Sea water, Air etc.

WHAT IS A MIXTURE?

It is a form of matter in which two or more elements or compounds combine physically in any proportion by weight.

Characteristics of Mixture

• Mixture may be homogeneous and heterogeneous.
• Mixture does not have a fixed melting point.
• In a mixture, the different constituents combine physically in any proportion by mass.
• The constituents of a mixture do not lose their identical property.
• Usually, no energy change take place during the formation of a mixture.

Types of Mixtures

• Homogeneous mixture: A mixture which has same composition throughout. It has no visible boundaries of separation between the various constituents Solutions are homogeneous mixtures. For example, Detergent in water, Sugar in water, Ice cream etc.
• Heterogeneous mixture: A mixture which has different compositions in different parts. These types of mixtures have visible boundaries of separation between the various constituents. For example, Oil in water, Fruit salad, Sand in water etc.

## 2. Impure Substance

### Impure substance

The substances that are formed by different kinds of particles are called impure substances. For example : Mixtures are impure substances.

Mixtures

You must have eaten pakoras at home. Have you noticed how your mother prepares it. She makes a batter containing besan, water, spices, etc. These constituents can be added in any ratio. In all ratios, they will form pakoras but the difference will be that sometimes, they will be hard to touch or the batter may be too loose to fry.

Mixtures: They are formed when two or more substances are simply mixed in any ratio and are not chemically combined with each other.

Characteristics of a mixture are as follows

• They can be homogeneous or heterogeneous in nature, that is, the constituents can be seen to have visible boundaries or they may appear to mix thoroughly.
• The properties of a mixture are the same as that of its constituents.
• The constituents can be separated by physical methods.
• Their formation does not require or release energy as there is no bond formation or breakage involved.
• The properties of mixtures like melting point & boiling point are not fixed.

Types of Mixtures

Solutions

We all like drinking sweet lemon water in summers as it gives a cooling effect. It is made of lemon, water, ice, salt and sugar. These all components when mixed, form sweet lemon water mixture which is also called a solution. These components can be separated by using different techniques. Let us learn what exactly solutions are and their types.

A Solution is a homogenous mixture of two or more substances. To form it, we can add 2 or more components. These components can be in any ratio but are simply mixed, that is not chemically combined. Generally, two components of solutions are seen and are called ‘Solute’ and ‘Solvent’.

Solute

It is the constituent which is present in a comparitively lesser amount and gets dissolved in the solvent.

Solvent

It is the constituent present in more amount and it has the ability to dissolve the solute in it. If you dissolve salt in water, then the salt is in lesser quantity and it gets dissolved. So, here solute is salt and solvent is water

Characteristics of solutions are as follows

• They are homogeneous.
• Their Composition can vary.
• The size of particles is very small.
• They do not scatter light.
• They can be separated by physical methods.

Types of solutions

1. On the basis of dissolving nature of liquids

You must have noticed that when you dissolve sugar or salt in water, it just vanishes after a few seconds or a minute. The reason is that it gets mixed in water. But if we add oil in water, it does not vanish and is seen floating on its surface. It is due to this reason that some substances can mix into each other and some do not. Let us learn about it in detail

Miscible, immiscible & partially miscible solutions

Miscible liquids

The liquids that completely mix into each other to form a solution are miscible liquids. For example: alcohol when added to water gets completely mixed.

Partially miscible liquids

The liquids which can dissolve in another liquid only up to some extent to form a solution are partially miscible liquids. For example: ethylene glycol in chloroform.

Immiscible liquids

The liquids which do not mix into each other are immiscible liquids. For example: oil & water.

2. On the basis of nature of solvent

We have seen that it’s not only water in which we can dissolve substances, we can make use of other substances as well. For example- carbon tetrachloride, Benzene, alcohol, etc. and many more reagents. So, we have another classification based on the nature of solvent.

Aqueous and Non aqueous solutions

Aqueous

The solution in which the solvent is water is an aqueous solution. Example: salt solution

Non-aqueous solutions

The solution in which the solvent is other than water is a non-aqueous solution. Example: alcohol, benzene etc.

3. Classification on the basis of solubility power of solvent

You must have seen that if you take one glass of water at room temperature and you add 1 spoon of sugar to it, it dissolves. But if you keep on adding sugar to the same solution, a point will be reached when it stops dissolving sugar in it and the sugar starts getting deposited at the bottom. The reason being that each solvent has some solubility power and it can dissolve only up to that limit. Let us study about it.

Saturated, unsaturated and supersaturated solutions

Saturated Solution

The solution that dissolves as much solute as it is capable of dissolving is a saturated solution.

Unsaturated solution

The solution in which more quantity of solute can be dissolved without increasing its temperature is an unsaturated solution.

Supersaturated

The solution in which the solvent dissolves an amount of solute greater than its solubility. It is formed at high temperature and then slowly cooling it to lower its solubility.

Solubility

Is the amount of solute that can be dissolved in a given amount of solvent at a particular temperature.

Factors affecting solubility

• Nature of solute.
• Nature of solvent.
• Temperature.

• On lowering temperature, solubility of liquids & solid decreases & solubility of gas remains unaffected.
• On increasing pressure, solubility of gas increases & for solid and liquid, it remains unaffected.

4. Classification on the basis of size of solute particles

You must have made a solution of sand in water, sugar in water and milk. They don’t look the same. Let us predict the nature of these solutions

Difference between a true solution, colloid and a suspension

## 3. Separation of Mixtures

Suspension and its properties

A suspension is a heterogeneous mixture in which the the solute particles do not dissolve but

remain suspended throughout the bulk of the medium. Ex: Chalk in water, smoke in the air.

Properties of Suspension :

1. It is a heterogeneous mixture.

2. Particles of a suspension are visible to the naked eye.

3. Size of the particles is greater than 100 nm.

4. It is unstable mixture. Solute settles down at the bottom over period of time.

5. If the solution is passed through filter paper, solute and solvent gets separated.

6. It scatters light when light is passed through the solution i.e. it shows Tyndall effect.

## 3. Separation of Mixtures

WHAT IS SOLUTION?

A solution is a homogenous mixture of two or more substances. E.g.  Nimboo pani, Soda water. A solution has a solvent and a solute as its components.
Solvent: The component of the solution that dissolves the other component in it is called the solvent.
Solute: The component of the solution that is dissolved is called the solute. in the solvent
Alloys: They are the mixtures of two or more metal or a metal and a non-metal and cannot be separated into their components by physical methods.

Properties of a Solution

•    A Solution is a Homogeneous mixture.
•    The particles of a solution are smaller than 1nm (10^-9 Metre) in Diameter. They cannot bean by Naked eyes.
•    They are of very Small Particle Size, so they do not scatter a beam of lighting passing through the solution.
•    The Solute Particles Cannot be separated from the mixture by the process of filtration.

Concentration of a solution

•    Saturated solution: Depending upon the amount of solute present in a solution, it can be called a dilute, concentrated or a saturated solution.
•    Unsaturated solution: If the amount of solute contained in a solution is less than the saturation level, it is called an unsaturated solution.
•    Solubility: The amount of the solute present in the saturated solution at this temperature is called its solubility.
•    The concentration of a solution is the amount of solute present in a given amount (mass or volume) of solution, or the amount of solute dissolved in a given mass or volume of solvent.
•    Concentration of solution = Amount of solute / Amount of solution
Or

Amount of solute / Amount of solvent

Ways of expressing the concentration of a solution

•    Mass by mass percentage of a solution

Mass of solute / Mass of solution x100
•    Mass by volume percentage of a solution

Mass of solute / Volume of solution x100

## 3. Separation of Mixtures

### Separation of Mixtures

1. To obtain colored component (dye) from Ink

Materials Required: Watch glass, Ink (blue/ black), Beaker, Stand, Burner

Procedure

1. Take a beaker and fill it half with water.

2. Take few ml of ink (Blue/ black) in the watch glass and place it on the mouth of the beaker.

3. Start heating the beaker and observe.

4. Heating is continued as long as the evaporation is taking place

5. Heating is stopped when no further change can be noticed on the watch glass

Observation

1. Evaporation taking place from the watch glass can be seen

2. Residue is left on the watch glass

Conclusion drawn

1. It can be concluded that,” Ink is not a pure substance but it is a mixture of dye in water which can easily be separated by evaporation method”.

2. Ink is not a single substance

3. Evaporation taking place from the watch glass containing ink.

Applications of evaporation process are as follows

1. This method can be used to separate the volatile component (solvent) from its non-volatile solute.

2. To obtain common salt from sea water. Sea water is trapped in small shallow pits (called lagoons) and is allowed to stand there for a few days. During this time the heat of the Sun evaporates water in these pits leaving behind the solid salt. This salt is further purified by crystallization method which you will study in the later part of this chapter.

2. To separate cream from milk by using centrifugation method.

Materials Required:  Full cream milk, Centrifuging machine/milk churner,  Jug test-tubes

Procedure

1. Take un boiled cold milk in two test-tubes and place these test-tubes in a centrifuging machine .

2. Centrifuge it at high speed by using a hand centrifuging machine for two minute and observe.

Note: If you don’t have centrifuging machine in your school then you can do this activity by using a milk churner used in the kitchen.

Observation:

Cream floating on the milk can be seen

Conclusion drawn:

When milk is rotated at high speed, then the suspended lighter particles (fats and protein molecules) bind with each other forming ‘cream’ and ‘skimmed milk’. The cream being lighter, floats over the skimmed milk which can be removed easily.

Applications of centrifugation method are as follows

1. This technique is used in washing machines to squeeze out water from wet clothes and make them dry.

2. This technique is used in diagnostic laboratories for testing cholesterol lipids in blood, ESR (Erythrocyte Sedimentation Rate) and various other types of tests.

3. It is used in dairies for separating cream from milk and butter from curd.

3. To separate kerosene oil from water by using a separating funnel.

Materials Required:  Separating funnel, Iron stand, kerosene oil, Beaker, Water.

Procedure

1. A mixture of kerosene oil and water is taken in a separating funnel. The separating funnel is clamped on an iron stand.

2. The mixture is allowed to stand so that it forms two distinct layers.

3. A clean beaker is placed below and the stop cock is opened slowly so that water falls into the beaker.

4. Close the stop- cock of the separating funnel as the oil reaches the stop cock.

Observation:

Kerosene oil being lighter forms the upper layer.

Conclusion drawn:

When a liquid mixture containing two immiscible liquids is taken in a separating funnel, then the liquid layers stand one above the other. The liquid layer with greater density forms the lower layer whereas the lighter liquid (having lower density) forms the upper layer.

Applications of this method are as follows

1. They separate mixture of oil and water.

2. In the extraction of iron from its ore, the lighter slag is removed from the top by this method to leave the molten iron at the bottom in the furnace.

4. To separate a mixture of common salt and ammonium chloride

Materials Required:  China dish, Tripod stand, Mixture of common salt and ammonium chloride, Glass funnel, Cotton, Burner.

Procedure

1. Take the mixture of sand and ammonium chloride in a china dish.

2. Cover the china dish with an inverted glass funnel and place it on a tripod stand.

3. Put a loose cotton plug in the opening of the funnel so as to prevent the escape of ammonium chloride vapours.

4. Heat the china dish on a low flame and observe.

Observation

1. White fumes (vapours) of ammonium chloride can be seen coming out of the mixture.

2. These white fumes start depositing as white solid, on coming in contact with the cold, inner walls of the funnel.

3. Sand salt is left behind in the china dish.

Conclusion drawn:

Ammonium chloride, being a volatile substance, changes into white vapours easily which deposit on the cold inner wall of the funnel. This ammonium chloride obtained and is called as Sublimate. Some other examples of solids which sublime are: Iodine, camphor, naphthalene and anthracene.

5. To separate the dyes (colored components) present in black ink.

Materials Required: Strip of filter paper, Water soluble ink ( sketch pen or fountain pen), Large size glass jar with lid, Cotton, Thread.

Procedure

1. Take a thin strip of filter paper (25cm x 5cm approx).

2. Draw a line on it using a pencil, approximately 3cm above the lower edge.

3. Put a small drop of water soluble black ink from a sketch pen or fountain pen) at the centre of the line. Let the ink dry.

4. Attach the paper strip on the thread with the help of cello tape.

5. Lower the filter paper strip into a large size gas jar in such a way that the drop of ink on the paper is just above the water level.

6. Adjust the thread and fix it on the sides of gas jar with the help of cello tape.

7. Cover the gas jar with a lid and leave it undisturbed.

8. Watch carefully as the water rises up on the filter paper.

9. Remove the filter paper strip, dry it and observe.

Observation:

Three colored spots can be seen on the filter paper strip.

Conclusion:

The given sample of black ink has three different dyes mixed in it.

6. To separate a mixture of two miscible liquids (acetone and water).

Materials Required:  Distillation flask, Thermometer, Condenser, Beaker, Iron stand, Mixture of acetone and water.

Procedure

1. Take the mixture in a distillation flask. Fit the flask with a thermometer.

2. Arrange the apparatus as shown in the figure.

3. Heat the mixture slowly, keeping a close watch at the thermometer and observe what happens.

Observation

1. The vapours of acetone can be seen rising up in the distillation flask with the increase in temperature.

2. When the temperature rises above 60o, the acetone gets vapourized and forms vapours in the flask.

3. These vapours get condensed in the condenser and can be collected (as pure liquid distillate) from the condenser outlet.

Result

Acetone can be collected in the beaker from the condenser outlet while water is left behind in the distillation flask.

7. To obtain crystals of pure copper sulphate salt from an impure sample by crystallization method.

Materials Required:  Impure sample of copper sulphate, Beaker, China dish, Glass rod.

Procedure

1. Take about 5 grams of impure sample of copper sulphate in a china dish.

2. Dissolve it in minimum amount of water.

3. Filter the copper sulphate solution to remove the insoluble impurities.

4. Heat the copper sulphate solution gently on a water bath until it becomes saturated and reaches its crystallization point.

5. Crystallization point can be checked by taking some solution on a glass rod and waving it in the air. When a solid film (small crystals) is formed on the glass rod, then further heating is stopped. This indicates that the solution has been concentrated to crystallization point.

6. Place the china dish on the table after covering it with a watch glass. Leave it undisturbed at room temperature to cool slowly for a day and observe thereafter.

Observation

Crystals of copper sulphate can be seen in the china dish along with the mother liquor (the residual liquid which is left after crystallization).

Conclusions drawn

Hot and concentrated solution of any pure substance forms crystal on cooling gradually.

Result:

Pure crystals of copper sulphate are separated from the mother liquor. These crystals are dried further between the folds of a filter paper or on a porous plate.

8. Purification of surface water

The purification of surface water can be done through following steps

1. Sedimentation: The river water is pumped into a series of sedimentation tanks where it is allowed to stay for a day. Heavy particles of clay and other impurities settle down at the bottom due to the effect of gravity. The supernatant water is then sent to another settling tank.

2. Loading: In next chamber, water is treated with chemicals, i.e. alum and lime to get further settling of impurities.

3. Filtration: The clear water from the second tank is pumped into filtration tank where sand and gravel filter completely remove the suspended impurities.

4. Chlorination: The clear water is chlorinated with a calculated amount of chlorine in the chlorination tank. This process kills harmful bacteria and germs and provides safe drinking water.

## 4. Physical And Chemical Changes

Colloidal solution and its properties

Colloid solution is heterogeneous mixture in which the size of particles lies between the true solutions and suspensions.

• Colloidal particles can easily scatter a beam of visible light. This phenomenon is called

Tyndall effect.

Properties of colloidal solution:

1. The particles of colloid can’t be seen by naked eyes individually.

2. It is a heterogeneous mixture and thus solute and solvent can’t be separated by filter paper.

3. Size of particles is smaller than suspensions but greater than solutions (1 nm to 100 nm).

4. It is a stable mixture. Particles do not settle down at the bottom over a period of time.

5. They do not settle down when left undisturbed which means colloid is quite stable.

Some common examples of colloids (in the table)

## 4. Physical And Chemical Changes

What is a suspension?

In which solids are dispersed in liquids, are called suspensions.
A suspension is a heterogeneous mixture
Particles of a suspension are visible to the naked eye.
Properties of a Suspension

•    Suspension is a heterogeneous mixture.
•    The particles of a suspension can be seen by the naked eye.

• The particles of a suspension scatter a beam of light passing through it and make its path visible.

•    The solute particles settle down when a suspension is left undisturbed, that is, a suspension is unstable.

WHAT IS A COLLOIDAL SOLUTION?

ïƒ˜    A colloidal solution is a heterogeneous mixture, for example, milk.
ïƒ˜    Because of the small size of colloidal particles, we cannot see them with naked eyes.
ïƒ˜    These particles can easily scatter a beam of visible light.
Tyndall effect
ïƒ˜    The scattering of a beam of light is called the Tyndall effect
ïƒ˜    The Tyndall effect can also be observed when a fine beam of light enters a room through a small hole.
ïƒ˜    This happens due to the scattering of light by the particles of dust and smoke in the air.

Observation of Tyndall effect

ïƒ˜    The Tyndall effect can be observed when sunlight passes through the canopy of a dense forest.
Properties of a colloid.
ïƒ˜    A colloid is a heterogeneous mixture.
ïƒ˜    The size of particles of a colloid is too small to be individually seen by naked eyes.
ïƒ˜    Colloids are large enough to scatter a beam of light passing through it and make its path visible.
ïƒ˜    They do not settle down when left undisturbed, that is, a colloid is quite stable.
ïƒ˜    They cannot be separated from the mixture by the process of filtration.

Dispersing medium

ïƒ˜    The components of a colloidal solution are the dispersed phase and the dispersion medium.
ïƒ˜    The solute – like component or the dispersed particles in a colloid form the dispersed phase, and the component in which the                dispersed phase is suspended is known as the dispersing medium.

## 4. Physical And Chemical Changes

Physical and chemical Change

We observe different kind of changes around us like if you forget to keep the milk inside the fridge in summers it gets rancid, likewise when you keep ice out of the refrigerator, you observe the water around it and finally the ice gets converted into water. These all are changes that are taking place around us. But some changes can be reversed like water can be converted back to ice but rancid food item can not be made fresh again. Let us learn in detail about these changes.

1. Physical change

2. Chemical change

## 5. What Are The Types Of Pure Substances ?

Separation of the components of mixtures

Different methods of separation are used to get from mixture.

Heterogeneous mixtures can be separated into their respective constituents by simple physical

methods like handpicking, sieving, filtration etc.

Obtaining coloured components from blue/black ink

Process of evaporation is used to obtain coloured components from blue/black ink. The process of

evaporation is used to separate a substance which is dissolved in water.

• It is based on the fact that liquid vaporises easily than the solid.

• Helps in separating volatile substances from non-volatile substances.

Steps of obtaining coloured components from blue/black ink:

• Fill half a beaker with water.

• Put a watch glass on the mouth of the beaker.

• Put few drops of ink on the watch glass.

• Now start heating the beaker. We do not want to heat the ink directly. You will see that evaporation

is taking place from the watch glass.

• Continue heating as the evaporation goes on and stop heating when you do not see any further

change on the watch glass.

Separation of cream from milk

• The process of centrifugation is used to separate the cream from milk. It is a method of separating

the suspended particles of substance from a liquid.

• This process is carried out by the machine called centrifuge.

• Sometimes, the solid particles in a liquid are very small and pass through a filter paper. For that

particles the filtration technique cannot be used.

• The mixture is rotated rapidly so that the heavier particles in the mixtures settle down to the

bottom.

• The basic principle of centrifugation is that the denser particles are forced to the bottom and

liquid being lighter remains at the top.

Steps of separating cream from milk:

• Take some full-cream milk in a test tube.

• Centrifuge it by using a centrifuging machine for two minutes.

Application of centrifugation:

• Used in diagnostic laboratories for blood and urine tests.

• Used in dairies and home to separate butter from cream.

• Used in washing machines to squeeze out water from wet clothes.

Separating two immiscible liquids

• The separation of separating two immiscible liquid is carried out by the use of funnel.

• The basic principle involve is the difference between the densities of two liquids form two separate layers.

Steps of separating kerosene oil and water:

• Pour the mixture of kerosene oil and water in a separating funnel.

• Let it stand undisturbed for sometime so that separate layers of oil and water are formed.

• Open the stopcock of the separating funnel and pour out the lower layer of water carefully.

• Close the stopcock of the separating funnel as the oil reaches the stop-cock.

Application of funnel:

• To separate mixture of oil and water.

• In the extraction of iron from its ore, the lighter slag is removed from the top by this method to

leave the molten iron at the bottom in the furnace.

Sublimation

• This process is used to separate mixtures that contain a sublimable volatile component from

non-sublimable impurity.

• Sublimation is process where a substance directly changes from solid to gaseous state on heating.

• Ammonium chloride, camphor, naphthalene and anthracene are some examples which can be

sublime.

Chromatography

• Used to separate those solutes which dissolve in the same solvent.

• Used for sepration of colours.

• The colours which are more soluble in water rises faster.

Applications

To separate

• colours in a dye

• pigments from natural colours

• drugs from blood.

Distillation

• Used for separation of components of a mixture containing two miscible liquids that boil with the

decomposition and have sufficient difference in their boiling points.

• Mixture of acetone and water is separated by this method.

Fractional distillation

• Fractional distillation is used to separate a mixture of two or more miscible liquids for which the

difference in boiling points is less than 25 K.

• Air is a homogeneous mixture and can be separated into its components by fractional distillation

Below is diagram which shows the steps of separation of air:

• The air is compressed by increasing the pressure and is then cooled by decreasing the temperature to get liquid air.

• The liquid air is warm-up slowly in a fractional distillation column, where gases get separated

different heights depending upon their boiling points.

• It used to separate a gas from the air.

Crystallization

• Used to remove impurities from solid and purify it.

• It separates a pure solid from mixture in the form of crystals.

• This process is used in purification of salt from sea water, separation of crystals of alum from

impure samples.

• It is better method than evaporation because:

(i) Solids decompose or some, like sugar, may get charred on heating to dryness.

(ii) Some impurities may remain dissolved in the solution even after filtration. On evaporation the

contaminate the solid.

## PHYSICAL & CHEMICAL CHANGES

Physical and Chemical changes

• The process which brings about changes in physical properties and no new substances are formed are physical changes. The common physical changes are changes in colour, hardness,rigidity, fluidity, density, melting point, boiling point etc.

• The process in which new Free substances are formed and chemical changed are chemical changes. Some chemical properties are odour, inflammability etc.

## PURE SUBSTANCES AND ITS TYPES

Types of pure substances

The pure substance is divided in two types on the basis of their chemical composition:

(i) Elements

(ii) Compounds

(i) Elements

• According to Antoine Laurent Lavoisier, element is a basic form of matter that cannot be broken

down into simpler substances by chemical reactions.

• It is divided in three types which are metals, non-metals and metalloids.

Properties of Metals

(i) They have a lustre (shine).

(ii) They have silvery-grey or golden-yellow colour.

(iii) They conduct heat and electricity.

(iv) They are ductile (can be drawn into wires).

(v) They are malleable (can be hammered into thin sheets).

(vi) They are sonorous (make a ringing sound when hit).

• Examples of metals are gold, silver, copper, iron, sodium, potassium etc.

• Mercury is the only metal that is liquid at room temperature.

Properties of non-metals

(i) They display a variety of colours.

(ii) They are poor conductors of heat and electricity.

(iii) They are not lustrous, sonorous or malleable.

Examples of non-metals are hydrogen, oxygen, iodine, carbon (coal, coke), bromine, chlorine

Metalloids: Elements having intermediate properties between those of metals and non-metal

called metalloids. Examples are boron, silicon, germanium etc.

(ii) Compounds

A compound is a substance composed of two or more elements, chemically combined with oneanother in a fixed proportion.

Difference between mixtures and compounds

## INTRODUCTION

Introduction

→ Around 500 B.C., Indian philosopher Maharishi Kanad, postulated the theory if we go on dividing

matter (padarth), we will obtain smallest particle beyond which further division can't be possibile

which is known as 'parmanu'.

→ Ancient Greek philosophers – Democritus and Leucippus called these particles atoms.

→ Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws

of chemical combination.

## 1. Dalton's atomic Theory

Introduction

→ Around 500 B.C., Indian philosopher Maharishi Kanad, postulated the theory if we go on dividing

matter (padarth), we will obtain smallest particle beyond which further division can't be possibile

which is known as 'parmanu'.

→ Ancient Greek philosophers – Democritus and Leucippus called these particles atoms.

→ Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws

of chemical combination.

Laws of Chemical Combination

• This law established after the experiments by Lavoisier and Joseph L. Proust.

• The chemical reaction between two or more substances give rise to products which is governed

by certain laws called Laws of Chemical Combination.

• Law of Conservation of Mass

→ During a chemical reaction, the total mass of reactants will be equal to the total mass of the products.

→ Mass can neither be created nor destroyed in a chemical reaction.

→ Example: A (reactant) + B (reactant) → AB (product)

mass of A + mass of B = mass of AB

• Law of Constant Proportions

→ In a chemical reaction, compounds always contain the same elements present in definite proportions by mass irrespective of their source.

→ It was given by Lavoisier.

→ For example:

(i) 18 gm of H2O = 2 gm of hydrogen + 16 gm of oxygen

⇒ mass of hydrogen/mass of oxygen = 2/16 = 1/8

(ii) 36 gm of H2O = 4 gm of hydrogen + 32 gm of oxygen

⇒ mass of hydrogen/mass of oxygen = 4/32 = 1/8

(iii) 9 gm of H2O = 1 gm of hydrogen + 8 gm of oxygen

⇒ mass of hydrogen/mass of oxygen = 1/8

This verifies law of constant proportions as the ratio of mass of hydrogen to oxygen is always same.

## 1. Dalton's atomic Theory

Chapter 3

Atoms & Molecules

Dalton’s Atomic Theory

The matter is made up of indivisible particles known as atoms.

The properties of all the atoms of a given element are the same, including mass. This can also be stated as all the atoms of an element have identical mass and chemical properties; atoms of different elements have different masses and chemical properties.

Atoms of different elements combine in fixed ratios to form compounds.

Atoms are neither created nor destroyed. The formation of new products (compounds) results from the rearrangement of existing atoms (reactants) in a chemical reaction.

The relative number and kinds of atoms are constant in a given compound.

Laws of Chemical Combination

Given by Lavoisier and Joseph L. Proust as follows:

Law of conservation of mass

According to the law of conservation of mass, matter can neither be created nor destroyed in a chemical reaction. It remains conserved.

Mass of reactants will be equal to the mass of products.

Law of constant proportions

A pure chemical compound contains the same elements combined together in a fixed proportion by mass is given by the law of definite proportions.

For e.g., If we take water from a river or from an ocean, both have oxygen and hydrogen in the same proportion.

Atom
Atoms are the smallest particles of an element which can take part in a chemical reaction.
Size of an atom: atomic radius is measured in nanometres.

The atomic symbol has three parts: -

The symbol X: the usual element symbol

The atomic number A: equal to the number of protons

The mass number Z: equal to the total number of protons and neutrons in an element.

## 1. Dalton's atomic Theory

Chapter-3

Atoms and Molecules

### Atoms and Molecules Introduction

We come across different things around us like chair, table, etc. and all the things that surround us have mass and weight. They all constitute matter. Matter is anything that occupies space and has mass. Matter is made up of small particles called atoms.

### Dalton's atomic Theory

Dalton’s atomic theory

According to Dalton’s Atomic Theory,

• All matter is made up of small particles called atoms.
• Atom is invisible.
• Atom is indivisible.
• Atoms of an element are alike in all aspects.
• Atoms of different elements combine in fixed whole number ratios to form compounds.

Atoms can neither be created nor destroyed.

Drawbacks of Dalton’s Atomic Theory were as follows

1. According to Dalton, an atom is indivisible but later on it was proved that atom can be subdivided into electrons, protons and neutrons.

2. Atoms of an element can somehow differ from each other.

So, these drawbacks led to the failure of Dalton’s theory of an atom. We also know that a lot of chemical reactions take place in our day to day life like making of tea, changing milk to curd, making cheese from milk and lots more. All the chemical reactions taking place obey certain set of laws. Let us study these chemical reactions and the rules that they obey.

Chemical reaction

The process by which some substances react to form a new substance. The substances which react are called reactants and the new ones formed are called products.

For example: A+B C+D

In this, A and B are reactants and C and D are called products.

For example: when we make tea, we add sugar, tea, milk and water. We mix them and heat the mixture. The result is that we get a new substance that is tea.

Laws of chemical combination

There are certain sets of laws that are obeyed by all chemical reactions. These laws are given by Lavoisier & Joseph. Let us study them in detail.

Law of conservation of mass

According to this law, ?matter can not be created nor destroyed in a chemical reaction.? That is, it always remains constant.

Like, in all chemical reactions the total mass of products is equal to the total mass of reactants.

For example:-  2H2 + O2        à                   2 H2O

(Reactants)                          (Product)

=> 4 + 2 x 16.32 2 x 18 => 36 g =  36 g.

This example shows that the total mass of reactants is equal to the total mass of products formed which is in accordance with the law.

Law of constant proportions: (Law of definite proportion)

According to it, in a chemical substance, the same elements are always present in a definite proportion by mass and volume irrespective of the method of preparation involved. For example, In Co2 molecule, the elements that form it remain the same that is carbon and oxygen and also, the ratio of C and O remains the same that is 3:8 by mass (i.e. 12.32) and 1 : 2 by volume.

Atom

It is the smallest particle of an element which may as may not have an independent existence. An atom is very small in size. The size is measured in unit of nm (nanometer). 1nm = 10^-9 m. Hydrogen atom is the smallest of all. Its size is only 10-10 nm. Atoms are represented by symbols (given by Berzelius).

## 2. IUPAC and atomic Symbols

Dalton's Atomic Theory

→ According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is

composed of small particles called atoms.

→ Six Postulates of Dalton's atomic theory:

(i) All matter is made of very tiny particles called atoms.

(ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.

(iii) Atoms of a given element are identical in mass and chemical properties. (Law of conservation of mass)

(iv) Atoms of different elements have different masses and chemical properties.

(v) Atoms combine in the ratio of small whole numbers to form compounds. (Law of constant proportion)

(vi) The relative number and kinds of atoms are constant in a given compound.

## 2. IUPAC and atomic Symbols

Atom
Atoms are the smallest particles of an element which can take part in a chemical reaction.
Size of an atom: atomic radius is measured in nanometres.
The atomic symbol has three parts: -
The symbol X: the usual element symbol
The atomic number A: equal to the number of protons
The mass number Z: equal to the total number of protons and neutrons in an element.
Atomic Mass

Atomic mass and atomic mass unit

Atomic mass is the total of the masses of the electrons, neutrons, and protons in an atom, or in a group of atoms, the average mass.
Mass of an atomic particle is called the atomic mass.
This is commonly expressed as per the international agreement in terms of a unified atomic mass unit (AMU).
It can be best defined as 1/12 of the mass of a carbon -12 atom in its ground state.
Molecule

It is the smallest particle of an element or a compound which can exist independently.
•    Molecules of an element constitute the same type of atoms.
•    Molecules may be monoatomic, diatomic or polyatomic.
•    Molecules of compounds join together in definite proportions and constitute a different type of atoms.

## 2. IUPAC and atomic Symbols

### IUPAC and atomic Symbols

IUPAC and Atomic symbols

IUPAC – International Union Of Pure & Applied Chemistry

The IUPAC approves the names of elements. According to it,

1. Symbol if an element is either the first letter of the name of the element or the first two letters of the name of an element.

2. In case there are two letters, then the first letter is in a capital case and the second letter is in small case.

For example- lead is written as “Pb” and not as “pb”. Sometimes, the symbol is derived from the Latin name of an element. For example, if we look at the symbol of Sodium- “Na” and not .So, because it is derived from its Latin name that is, “Natrium”. Similarly, for Potassium, Copper, Iron, Mercury, etc. Latin names and symbols of a few elements are given below:

The atomic mass of the element

If we try to find the exact mass of an atom, we can’t find it. The reason being that an atom is too small in size that it is difficult to measure its size and moreover, the size of an atom changes, depending upon its neighbouring- atoms. Therefore, instead of finding its absolute mass, we try to find the relative mass of an atom. The standard element chosen for finding relative mass is carbon-6.

The relative atomic mass of an atom of an element is defined as the average relative mass of an atom as compared to an atom of 612c taken as 12u.

Atomic mass: In simple language, atomic mass is the number of times an atom of an element is heavier than 1/12th of a Carbon atom. For example: If we say that the atomic mass of Sodium is 23, it means that sodium is 23 times heavier than 1/12th of a carbon atom.

Molecule

The particle next to an atom is a molecule, but it has an independent existence and is formed when atoms combine with each other. Let us study about it. A Molecule is the smallest particle of an element or compound which is able to exist independently. For example, carbon dioxide molecule, hydrogen gas molecule, etc . But it has been seen that some molecules are formed by the combination of two atoms of the same kind like H2 and some may have two or more atoms of different kinds like H2O. This shows that in a molecule any number of atoms can be present and they may be of the same or different kinds.

Atomicity: It is the number or the kinds of atoms present in a molecule of an element. Some more examples: Phosphorous exists as P4 (tetra atomic), Sulphur exists as S8 (polyatomic), Nitrogen exists as N2 (diatomic).

## 3. Compound

Atoms

• Atoms are building blocks of all matter.

• According to modern atomic theory, an atom is the smallest particle of an element which taking

part in chemical reaction.

• Atoms are very small and which can’t be seen even through very powerful microscope.

• Atomic radius is measured in nanometres. Nanometres = 10-9 m.

• Modern day symbols of Elements

→ Dalton was the first scientist to use the symbols for elements.

→ Berzilius suggested that the symbols of elements should be made from one or two letters of

name of the element.

→ The name copper was taken from Cyprus, a place from where it was found for first time.

→ Now, IUPAC (International Union of Pure and Applied Chemistry) approves names of element

→ The first letter of a symbol is always written as a capital letter (uppercase) and the second letter.

as a small letter (lowercase). For example: hydrogen (H), aluminium (Al), cobalt (Co).

→ Some other symbols have been taken from the names of elements in Latin, German or Greek.For

example: Fe from its Latin name ferrum, sodium is Na from natrium, potassium is K from kalium.

• Atomic Mass

→ Dalton’s atomic theory proposed the idea of atomic mass which explained the law of constant

proportions so well.

→ The mass of an atom of an element is called its atomic mass.

→ In 1961, IUPAC have accepted ‘atomic mass unit’ (u) to express atomic and molecular mass of

elements and compounds.

→ The atomic mass unit is defined as the quantity of mass equal to 1/12 of mass of an atom of

carbon-12.

1 amu or u = 1/12 × Mass of an atom of C -12

1 u = 1.66 × 10-27 kg

• Atom existence

→ Atoms of most of the elements are very reactive and does not exist in free state.

→ Only the atoms of noble gases (such as He, Ne, Ar, Kr, Xe and Rn) are chemically unreactive and

can exist in the free state as single atom.

→ Atoms of all other elements combine together to form molecules or ions.

## 3. Compound

Atomicity
The number of atoms constituting a Molecule is known as its atomicity.

Compounds

A pure substance made up of two or more elements chemically combine together in a fixed ratio under fixed condition is called compound. Example: Calcium carbonate, Common salt, Sugar.

Properties of compound

1.Compounds can be separated into the constituent only by chemical methods.
2. Properties of compound differ from the properties of their constituents.
3. During the formation of a compound energy is absorbed or released.
4. The compound are homogeneous.

Valency

The combining capacity of an element is known as its valency. Valency is used to find out how the atom of an element will combine with the atom of another element to form a chemical compound.
(Every atom wants to become stable, to do so it may lose, gain or share electrons.)

•    If an atom consists of 1, 2 or 3 electrons in its valence shell then its valency is 1, 2 or 3 respectively,
•    If an atom consists of 5, 6 or 7 electrons in the outermost shell, then it will gain 3, 2 or 1 electron               respectively and its valency will be 3, 2 or 1 respectively.
•    If an atom has 4 electrons in the outermost shell than it will share this electron and hence its valency         will be 4.
•    If an atom has 8 electrons in the outermost electron and hence its valency will be 0.

Ions
An ion is an electrically charged atom or group of atoms.
An ion is formed by the loss or gain of electrons by an atom, so it contains an unequal number of electrons and protons.

There are two types of ions:

Cation: A positively charged ion is known as cation.
For Ex: Na+, Mg2+
A cation is formed by the loss of one or more electrons by an atom.

Na – e– ——–>Na+
Z=11                 Z=10
2,8,1                  2,8
K, L, M                  K, L

Mg – e– ——–> Mg2+
Z=12                 Z=10
2,8,2                  2,8
K,L,M                  K,L

Anion: A negatively charged ion is known as anion.
For Ex: Cl–, O2-
An anion is formed by gain of one or more electron by an atom.

Cl + e– ——–> Cl–
Z=17                 Z=18
2,8,7                  2,8,8
K, L, M                 K, L, M

O + e– ———> 02-
Z=8                   Z=10
2,6                    2,8
K, L                    K, L

An atom or group of atoms having a charge, i. e. either negative or positive, on it.
The radicals having positive charge are called cations.eg. Sodium ion.
The radicals having negative charge are called anions. eg. Chloride ion.

## 3. Compound

### Compound

Compound

When atoms of two or more different elements combine together in a definite proportion by mass and volume, it forms a compound. for example:- H2O is formed only when the ratio of H and O is 2:1 by volume and ratio by mass is 1:8.

Ion

We know that the atom is neutral, that is, the number of positive and negative charges in it is equal but if sometimes, an atom gains or loses negative or positive charge, it no longer remains neutral. It becomes an “ion”. Let us study about it.

Ion:- The atom that bears a charge is called an “ion”. Ion is of two types:

1. Cation

2. Anion

Cation: they are positively charged ions. They are formed when an atom loses a negative charge. Therefore, they always have negative charges less than normal atoms. Example: Na -1 negative charge Na+ (cation)

Anion: they are negatively charged ions and are formed when an atom gains a negative charge. Therefore, they have negative charges more than normal atoms. Example: S +2 negative charges S2- (anion)

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The group of atoms that bears a charge is also called a polyatomic ion. The following radicals are given below:

## 4. Chemical Formula

Molecules

→ A molecule is in general a group of two or more atoms that are chemically bonded together.

→ A molecule is the smallest particle of matter (except element) which is capable of an independent existence and show all properties of that substance.

→ Examples: ‘H2O’ is the smallest particle of water which shows all the properties of water.

→ A molecule may have atom of same or different elements, depending upon this, molecule can be categorized into two categories:

(i) Homoatomic molecules (containing atom of same element)

Examples: H2, O2, O3 , S8 , P4 etc.

(ii) Heteroatomic molecules or compounds (containing atoms of different elements)

Examples: H2O, CO2 , NaCl, CaCO3 etc.

## 4. Chemical Formula

Chemical Formulae

Rules:

(i) The valencies or charges on the ion must balance.

(ii) Metal and non-metal compound should show the name or symbol of the
metal first.
e.g., Na+ Cl– → NaCl

(ii) If a compound consists of polyatomic ions. The ion is enclosed in a bracket before writing the number to indicate the ratio.
H1+ SO42- → H2SO4

Molecular Mass
It is the sum of the atomic masses of all the atoms in a molecule of the substance. It is expressed in atomic mass unit (u).

Formula Unit Mass
It is the sum of the atomic masses of all atoms in a formula unit of a compound. The constituent particles are ions.

Mole Concept

Definition of mole: It is defined as one mole of any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams.
1 mole = 6.022 x 1023 in number
Molar mass = mass of 1 mole → is always expressed in grams and is also known as gram atomic mass.
l u of hydrogen has → 1 atom of hydrogen and 1g of hydrogen has → 1 mole of hydrogen
= 6.022 x 1023 atoms of hydrogen.

## 4. Chemical Formula

### Chemical formula

Chemical Formula is the symbolic representation of compound. For example, the compound ammonium nitrate is written as

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If we say that water is represented as H2O, we get the following information from it

1. H2O represents water.

2. H2O represents one molecule of water.

3. It represents that 1molecule of H2O contain 2Hydrogen and 1 Oxygen atoms.

4. It represents 18g of water.

5. It represents that 1 molar mass of water has 6.022 x 10 23 molecules.

Mass percentage

Mass Percentage is the ratio of mass of a substance whose percentage is to be calculated to the total mass of the substance multiplied by 100.

Example 1: Find the mass percentage of 6 g sodium hydroxide dissolved in 50 g of water. (Note: since the density of water is nearly 1, this type of question often gives the volume of water in milliliters.)

First find the total mass of the solution:

total mass = 6 g sodium hydroxide + 50 g water
total mass = 56 g

Now, you can find the mass percentage of the sodium hydroxide using the formula:

mass percent = (grams of solute / grams of solution) x 100
mass percent = (6 g NaOH / 56 g solution) x 100
mass percent = (0.1074) x 100

Example 2: Find the masses of sodium chloride and water required to obtain 175 g of a 15% solution.

This problem is a bit different because it gives you the mass percentage and asks you to then find how much solute and solvent are needed to yield a total mass of 175 grams. Start with the usual equation and fill in the given information:

mass percent = (grams solute / grams solution) x 100
15% = (x grams sodium chloride / 175 g total) x 100

Solving for x will give you the amount of NaCl:

x = 15 x 175 / 100
x = 26.25 grams NaCl

So, now you know how much salt is needed. The solution consists of the sum of the amount of salt and water. Simply subtract the mass of salt from the solution to obtain the mass of water that is required:

mass of water = total mass - mass of salt
mass of water = 175 g - 26.25 g
mass of water = 147.75 g

Mole Concept

Mole Concept is equal to 6.023 x 1023 entities present in a substance. It is also called Avagadro’s number. Mole = given mass of substance to its atomic or molecular mass.

Example 1: Calculate the number of moles for the following:

(i) 52 g of He (finding mole from mass)
(ii) 12.044 × 1023 number of He atoms (finding mole from number of particles).

Solution:

No. of moles = n

Given mass = m

Molar mass = M

Given number of Particles = N

Avogadro number of Paticles = N0

(i) Atomic mass of He = 4u

Molar mass of He = 4g

Thus, the number of moles = given mass/molar mass

n = m/M = 52/4 = 13

(ii) we know,

1 mole = 6.022 × 1023

The number of moles

= given number of particles/Avogadro number

n=N/N0= 12.044 × 1023/6.022 × 1023= 2

Example 2: Calculate the mass of the following:

(i) 0.5 mole of N of molecule)

(ii) 0.5 mole of N atoms (mass from mole of atom)

(iii) 3.011 × 1023 number of N atoms (mass from number)

(iv) 6.022 × 10 23 number of N2 molecules (mass from number)

#### Solution:

(i) mass = molar mass × number of moles
m =  M x n =  28 0.5 = 14 g

(ii) mass = molar mass × number of moles
m = M × n = 14 × 0.5 = 7 g

(iii) The number of moles, n

= given number of particles/Avogadro number =N/N0

= 3.011 × 1023/6.022 × 1023

m=M x n =14 x 3.011 × 1023/6.022 × 1023

=14×0.5 =7 g

(iv) n =N/N0

m= M x N/N0= 28 x 6.022 × 1023/6.022 × 1023

=28×1=28 g

## 5. Molecular Mass And Mole Concept

Atomicity

→ The number of atoms present in one molecule of an element is called its atomicity.

## CHEMICAL FORMULAE

Chemical Formulae

→ It is the symbolic representation of the composition of a compound.

• Characteristics of chemical formulae

→ The valencies or charges on ion must balance.

→ When a compound is formed of metal and non-metal, symbol of metal comes first. E.g., CaO

NaCl, CuO.

→ When polyatomic ions are used, the ions are enclosed in brackets before writing the numbe

show the ratio. E.g., Ca(OH)2, (NH4)2SO4

• Rules for writing chemical formulae

(i) We first write symbols of elements which form compound.

(ii) Below the symbol of each element, we should write their valency.

(iii) Now cross over the valencies of combining atoms.

(iv) With first atom, we write the valency of second atom (as a subscript).

(v) With second atom, we write valency of first atom (subscript).

## MOLECULAR MASS

Molecular Mass

→ It is the sum of atomic masses of all the atoms in a molecule of that substance.

Example: Molecular mass of H2O = 2×Atomic mass of Hydrogen + 1×Atomic mass of Oxygen

So, Molecular mass of H2O = 2×1 + 1×16 = 18 u

Formula Unit Mass

→ It is the sum of atomic mass of ions and atoms present in formula for a compound.

Example: In NaCl,

Na = 23 a.m.u.

Cl = 35.5 a.m.u.

So, Formula unit mass = 1×23

+ 1×35.5 = 58.5 u

## IONS AND MOLE CONCEPT

Ions

→ An ion may be defined as an atom or group of atoms having positive or negative charge.

→ Some positively charged ions : Na+ , K+ , Ca2+ , Al3+

→ Some negatively charged ions : Cl- (chloride ion), S2- (sulphide ion), OH-(hydroxide ion),

SO42- (sulphate ion)

• We can classify ions in two types:

(i) Simple ions

Mg2+ (Magnesium ion)

Na+(Sodium ion)

Cl-(Chloride ion)

Al3+ (Aluminium ion)

(ii) Compound ions

NH4+ (Ammonium ion)

CO32- (Carbonate ion)

SO42- (Sulphate ion)

OH- (Hydroxide ion)

• Chemical Formulae of Ionic Compounds (Polyatomic)

Mole Concept

→ A group of 6.022×1023 particles (atoms, molecules or ions) of a substance is called a mole of

substance.

→ 1 mole of atoms = 6.022×1023 atoms

→ 1 mole of molecules = 6.022 × 1023 molecules

Example, 1 mole of oxygen = 6.022×1023 oxygen atoms

Note: 6.022×1023 is Avogadro Number (L).

→1 mole of atoms of an element has a mass equal to gram atomic mass of

the element.

Molar Mass

→ The molar mass of a substance is the mass of 1 mole of that substance.

→ It is equal to the 6.022×1023 atoms of that element/substance.

Examples:

(a) Atomic mass of hydrogen (H) is 1 u. Its molar mass is 1 g/mol.

(b) Atomic mass of nitrogen is 14 u. So, molar mass of nitrogen (N) is 14 g/mol.

(c) Molar mass of S8 = Mass of S×8 = 32×8 = 256 g/mol

(d) Molar mass of HCl = Mass of H + Mass of Cl = 1 = 35.5 = 36.5 g/mol

Important Formulae -

## INTRODUCTION

Introduction

→ John Dalton considered atom to be an indivisible entity, but his concept had to be discarded the end of nineteenth century,through experiments were able to find existence of  charged (electrons and protons) and neutral particles (neutrons) in the atom. These particles are called the ‘Sub-atomic Particles’.

## 1. Introduction

Discovery of Electrons – Cathode Rays (By J. J. Thomson)

→ Thomson explained presence of electrons by cathode rays experiment.

→ Charge on electron = −1.6 × 10-19 C (C = Coloumb)

(As calculated by Robert E. Millikan)

→ Mass of electron = 9.1 × 10-31 kg

## 1. Introduction

Chapter 4

Structure of an Atom

Charged particles in Matter

•    Atoms are the basic building blocks of matter.
•    Different kinds of matter exist because there are different kinds of atom
present in them.

Postulates of Dalton’s Atomic Theory

•    All matter is made up of tiny, indivisible particles called atoms.
•    All atoms of a specific element are identical in mass, size, and other properties. However, atoms of different element exhibit different properties and vary in mass and size.
•    Atoms can neither be created nor destroyed. Furthermore, atoms cannot be divided into smaller particles.
•    Atoms of different elements can combine with each other in fixed whole-number ratios in order to form compounds.
•    Atoms can be rearranged, combined, or separated in chemical reactions.
•    John Dalton considered atom to be an individual entity, but his concept had to be discarded                  at the end of 19th century, when scientist through experiments able to find existence of charge (electrons & protons) and neutral particles (neutrons) in the atom. These particles were called sub atomic particles.
•    The discovery of electron and proton is credited to J.J. Thomson and E. Goldstein, respectively.
•    J.J. Thomson proposed that electrons are embedded in a positive sphere
•    Electron was represented as 'e-' and proton as 'p+'. The mass of a proton is taken as one unit and its charge as plus one where the mass of an electron was considered to be negligible and its charge minus one
•    It seemed that atom consisted of electrons and protons which balanced their charges mutually.

## 1. Introduction

Chapter-4

Structure of the Atom

### Structure of the Atom Introduction

As you all know, we come across different substances around us that constitute matter. This matter is made up of small particles called atoms. Let us know more about atom, its structure and its constituents. Matter: is anything that occupies space and has mass. It consists of tiny particles which were called ‘parmanu’ but later on this name was replaced by ‘atom’.

Dalton’s Atomic Theory

The scientist, Dalton was the first one to actually formulate all information of an atom in a theoretical form as Dalton’s Atomic Theory. He was born in 1766 and died in 1844. His theory was published in 1808. His full name was John Dalton

According to this theory, the following observations were made

1. All matter is made up of small particles called atoms.

2. Atom is invisible.

3. Atom is indivisible.

4. Atoms of an element are alike in all aspects, that is, if we talk about sodium then all the atoms of sodium will be the same in all aspects.

5. Atoms of different elements are different, that is, if we talk about sodium and potassium, then, the atoms of both are going to be different but same among themselves.

6. Atoms of different elements combine in a fixed simple whole number ratio to form compounds.

7. Atoms can neither be created nor destroyed, that is, the origin of atoms is not known.

Drawbacks of Dalton’s Atomic Theory

There were certain limitations as observed by other scientists. They observed the following

1. According to Dalton, an atom was indivisible but later on, it was proved that atom can be subdivided into sub atomic particles called electrons, protons & neutrons.

2. Atoms of the same element can somehow differ from each other. This was proved due to the existence of isotopes in nature.

3. Similarly, atoms of different elements can be the same. This came into notice due to the existence of isobars in nature.

4. According to it, whenever the compound is formed, it is formed as a result of the combination of atoms in a fixed simple ratio. But it has been seen that the ratio might not always be simple. For example, in sucrose that is C12H22O11, the ratio is not a simple ratio.

So, these drawbacks led to failure of Dalton’s theory of an atom.

## 2. Discovery of Electrons and Protons

Discovery of Protons – Anode Rays/Canal Rays (By E. Goldstein)

→ E. Goldstein by his famous anode rays/canal rays experiment was able to detect presence of positively charged particles called protons in the atom.

→ Charge on proton = + 1.6 × 10-19 C

→ Mass of proton = 1.673 × 10 -24 gm

i.e., Mass of proton ≅ 1840 × Mass of electron

## 2. Discovery of Electrons and Protons

The Structure of an atom

• Dalton's atomic theory suggested that the atom was indivisible and indestructible. However, the discovery of two fundamental particles in the atom the electrons and protons led to the failure of this aspect of the theory.
• Hence, J.J. Thomson was the first to propose a model for the structure of an atom.

Thomson’s Model of an Atom

According to J.J. Thomson, the structure of an atom can be compared to Christmas pudding where electrons are present inside a positive sphere.

•    An atom is composed of a positively charged sphere in which electrons are embedded.
•    Atom is neutral as the positive and negative charged are equal in proportion.
Rutherford’s Model of an Atom

Rutherford’s Experiment

•    He experimented with thin gold foil by passing alpha rays through it.
•    He expected that the gold atoms will deflect the Alpha particles.

 Observations Inferences Alpha particles which had high speed moved straight through the gold foil Atom contains a lot of empty space Some particles got diverted a by slide angles Positive charges in the atom are not occupying much of its space Only one out of 12000 particles bounced back The positive charges are concentrated over a particular area of the atom.

Thus, Rutherford gave the nuclear model of an atom based on his experiment which suggests that -

•    Atoms contain a lot of unoccupied space
•    There is a heavily positively charged substance present in the center of the atom which is called the          nucleus
•    The nucleus contains an equal amount of positive and negative charge.

### Drawbacks of the Nuclear Model of an Atom

•    The Nuclear Model of the Atom failed to explain how an atom remains stable despite having positive and negative charges present in it.
•    Maxwell has suggested a theory according to which if any charged particle moves in a circular motion, it radiates energy.
•    So, if electrons start moving in a circular motion around the nucleus, they would also radiate some energy which would decrease at the speed of the electrons.
•    As a result, they would fall into the nucleus because of its high positive charge.

What are nucleons? –  Protons and Neutrons are collectively called as Nucleons.

## 2. Discovery of Electrons and Protons

### Discovery of Electrons and Protons

Discovery of electrons

It was discovered by J.J. Thomson in the year 1897. He took Crooke’s tube and arranged the apparatus as shown in figure:

1. When he passed electric current (at high voltage approximately at 10,000volts) through a gas at a pressure of 1 atm, then nothing happened as no changes were seen.

2. When he reduced the pressure to 10-2 atm, the whole tube started glowing with green colour.

3. He further reduced the pressure to 10-4 atm, the whole tube stopped glowing, but a faint green colour was still seen at the anode end.

4. To confirm, a fluorescent screen was placed at the back of the anode and anode was made perforated. When current was passed through it (in the same physical conditions), the Zinc Sulphide screen started glowing which confirmed the following fact.

The following was concluded

It proved that at these conditions, some rays were emitted through cathode and were travelling towards anode called cathode rays consisting of negatively charged particles. These particles were later called electrons. This is how the electron was discovered.

Discovery of Protons

It was discovered by E.Goldstein in the year 1920. It was actually discovered while carrying out an experiment to produce cathode rays. The apparatus used was the same:

It was seen that when cathode rays were produced (at high voltage and low pressure), they travelled through the gas in the discharge tube. While doing so, they ionized the gas. That is, they took electrons of gas along, leaving behind positively charged particles of the gas. These particles formed canal rays and started moving towards the cathode. These particles are called canal rays as they are not produced by anode. As these particles possess positive charge so they were named as protons.

Thomson’s model of an atom (plum pudding model)

His full name was J.J. Thomson and he was the one who made the first attempt to explain the structure of atoms.

According to him, an atom is a positively charged sphere in which negative charges are present at certain places like plums in a pudding or cherries in an ice-cream. But this model was rejected as he could not explain the major point seen in his model.

Drawback of Thomson’s atomic model were as follows

He could not explain the distribution of charges and stability of an atom. As we all know that opposite charges attract each other, so, how come it is possible that few negative charges remain scattered in this big positive space. They would have been neutralized. This could not be explained by J.J. Thomson that lead to failure of his attempt.

Rutherford’s scattering experiment

In order to understand the structure of an atom, Rutherford performed the scattering experiment. For this, he took a gold foil and passed alpha rays through it. Gold foil actually consists of many gold atoms. So, at an individual level, we are considering the observations through an atom. Alpha rays are actually positively charged rays consisting of Helium nucleus (He).

These rays, when passed through it suffered reflections at different angles and to note that a movable screen made of fluorescent material was placed around it. When the reflected rays strike that screen it causes scintillation. When he passed these rays through gold foil various observations were seen.

Observations were as follows

• Most of the rays passed straight.
• Some rays were deflected through small & large angles.
• Some rays rebound back.

• Most of the space in an atom is empty.
• There is something in the centre of an atom called nucleus.
• Nucleus is positively charged.

So, According To Rutherford, the structure of an atom is similar to that of the solar system.

He said:

• Atom is electrically neutral
• Nucleus is in centre in which protons are present.
• Outside nucleus electrons revolve like planets revolve around the Sun.

In this model also few limitations were seen and some questions were left unanswered, which led to its failure.

The drawback of Rutherford’s experiment were as follows –

He failed to explain the stability of an atom”.

According To electromagnetic theory: Any charged particle, when revolves in a circular path, continuously emits energy and shortens its path. As we know, an electron is also a charged particle revolving in a circular path so it should also emit energy, shorten its path and should finally falls into the nucleus and for doing so, it needs time lesser than a fraction of a second. But this doesn’t happen.

## 3. Discovery of Neutrons

Discovery of Neutrons (By J. Chadwick)

J. Chadwick bombarded lighter elements (like lithium, boron etc.) with α-particles and observes

emission of new particles having zero charge but having mass equal to that of proton.

→ These particles were called ‘Neutron’ i.e., neutral particle of the atom.

→ Neutron are absent in Protium isotope of hydrogen atom.(1H1)

→ Since, mass of electrons are negligible as compared to that of proton and neutrons hence, sum

of masses of protons and neutrons in an atom will compose its atomic mass.

## Bohr Model of an atom

Bohr suggested that –

•    Electrons spin around the nucleus in an individualized separate path or unattached          orbit.
•    The electrons do not emit any energy while moving Indies special orbits.
•    These orbits are also called as Energy Levels.
•    They are represented using letters or numbers as shown in the figure below –

The Neutrons

J. Chadwick discovered that there is another sub-atomic particle present in the atom. This particle carries no charge and is known as a Neutron. Therefore, we can conclude that atom consists of three types of particles –

 Electrons which carry a negative charge Protons which carry a positive charge Neutrons they are neutral

### The distribution of electrons in different shells or orbits

•    If Orbit number = n
•    Then number of electrons present in an Orbit = 2n2
•    So, for n =1
•    Maximum electrons present in shell – K = 2 * (1)2 = 2
•    The outermost shell can contain at most 8 electrons.
•    The shells in an atom are filled in sequence.
•    Thus, until the inner shells of an atom are filled completely the outer shells cannot         contain any electrons.

## 3. Discovery of Neutrons

### Discovery of Neutrons

Discovery of neutron

It was discovered by Chadwick in the year 1932. It was actually discovered while considering the mass of atomic particles. It was seen that the whole mass of an atom is due to the nucleus and as far we know, the nucleus is positively charged. That means it has only protons in it. But we also know that the mass of an atom is never equal to the number of protons. This shows that the nucleus contains some other particles also that contribute towards mass only and not towards charge. Therefore, the particles were called neutrons (as they possess no charge).

Bohr’s theory

To overcome the limitations of Rutherford model, the new concept and picture of atom was given by Neil Bohr which made a great contribution in knowing the structure of an atom. According to it:

• Atom is electrically neutral i.e. number of Protons = number of Electrons.
• In the centre of an atom, nucleus is present which is positively charged.
• In the nucleus, protons and neutrons are present.

Protons possess = positive

Neutron possess = no charge

• Outside the nucleus, shells or energy levels designated as K,L,M,N and so on  are present.
• In shells, electrons revolve.
• Electrons are negatively charged.
• Each shell has a fixed amount of energy. So, as long as an electron remains in the same shell, it never loses or gains energy.
• Number of electrons  in each shell is determined by Bohr Bury rule i.e. 2n2.

## 4. Some Important Defintion

Atomic Models

→ From the knowledge of existence of subatomic particles like electron, proton and neutron in

atom, various atomic models were proposed by different scientists.

• Some of the atomic models:

(i) Thomson’s Model of Atom

(ii) Rutherford’s Model of Atom

(iii) Bohr’s Model of Atom

→ The most trusted and scientifically established model of atom which is adopted these days

‘Quantum Mechanical Model of Atom’. It will be dealt in higher classes.

Thomson’s Atomic Model

→ This model is often called the ‘Water Melon Model’.

→ In this model, Thomson predicted the presence of electrons inside positive sphere (made up

protons), just same as seeds of watermelon are embedded in red edible part of watermelon.

→ Although this model explained neutrality of atom but couldn’t able to explain other scientific

experiments conducted on atom. Hence it was discarded.

## 4. Some Important Defintion

Valency

•    Valence Electrons – Electrons existing in the outermost orbit of an atom are called Valence Electrons.

•    The atoms which have completely filled the outermost shell are not very active chemically.
•    The valency of an atom or the combining capacity of an atom is given by the number of elements present in the outermost shell.

•    For Example, Helium contains two electrons in its outermost shell which means its valency is two. In other words, it can share two electrons to form a chemical bond with another element.

•    What happens when the outermost shell contains a number of electrons that are close to its maximum capacity?

Valency in such cases is generated by subtracting the number of electrons present in the outermost orbit from octet (8). For example, oxygen contains 6 electrons in its outermost shell. Its valency is calculated as: 8 – 6 = 2. This means oxygen needs two electrons to form a bond with another element.

Atomic Number of an Element

Atomic Number (Z) = Number of protons in an atom
Mass Number of an Element
Mass Number = Number of protons + Number of neutrons

## Isotopes

•    The atoms of an element can exist in several forms having similar atomic numbers but varying mass numbers.
•    Isotopes are pure substances.
•    Isotopes have a similar chemical nature.
•    Isotopes have distinct physical characteristics.

### Where can we use Isotopes?

1. The fuel of Nuclear Reactor – Isotope of Uranium
2. Treatment of Cancer – Isotope of Cobalt
3. Treatment of Goitre – Isotope of Iodine

Isobars

The atoms of several elements can have a similar mass number but distinct atomic masses. Such elements are called Isobars.

Isotones

•    Species having same number of neutrons but different number of protons are called Isotones.

•    Examples include boron-12 and carbon-13 nuclei both contain 7 neutrons, and so are isotones.

Difference Between Isotopes, Isobars & Isotones

## 4. Some Important Defintion

### Some Important Defintion

Atomic number (z): Is defined as “Number of protons in atom”.

Mass number (A): Is defined as “sum of protons and neutrons present in the nucleus of an atom.”

Representation of an element

Where Z = atomic number (Sub script)

Where A= mass number (Superscript)

Electronic configuration It is defined as “Arrangement of electrons in different shells.” The arrangement is according to Bohr bury rule that is 2n².

Valence shell It is the last shell of an atom. For example, in the case of Sodium, the electronic configuration is 2,8, 1. In this, the shells involved are K,L,M. Therefore, the last shell is M (valence shell).

Valence electrons The electrons present in the valence shell of an atom, for example, in the case of Sodium, the electronic configuration is 2,8, 1. In this, the shells involved are K,L,M. The electrons present in valence shell are valence electrons that are 1.

Valency It is the combining capacity of an atom. It depends upon the number of valence shell electrons that is electrons in the last shell of an atom (that can be seen while writing electronic configuration). We have two types of valency that is:

• Electro-valency
• Covalence
• Electro-valency: It is the valency that is attained by losing and gaining of electrons. For example, in the case of Sodium, the atomic number is 11 and electron distribution is 2,8,1. So, in order to attain stability, it can either lose one electron or gain 7 electrons.  Out of the two oprions, losing 1 electron is easier, therefore, it loses one electron and therefore, its valency is +1.
• Covalence: It is the valency that is attained by sharing of electrons. For example, in the case of carbon, it can attain stability by sharing 4 electrons. If we look at its distribution, it needs four more electrons, so it completes its octet by sharing four electrons. Therefore, its valency is -4.

Isotopes

They are those elements which have the same atomic number but different mass numbers.

Properties of isotopes are as follows

• The number of protons is the same.
• The symbol is the same.
• Number of valence electrons and valency is same.
• They differ only in number of neutrons.

Applications of isotopes

• U-235 is used as a nuclear fuel.
• Co-60 used in the treatment of cancer.
• I-128 used in the treatment of goitre.
• C-14 is used in carbon dating.

For example:

• In the case of Hydrogen, the isotopes are – H1  H2  H3 .  (But atomic number for all is 1).
• In case of oxygen, the isotopes are – O16  O17  O18 . (But atomic number for all oxygen is 8).

Isobars

They are those elements which have the same mass number but different atomic number. For example: sodium and magnesium are isobars as they have different atomic numbers but same mass number that is 24.

Isotones

They are those elements which have the same number of neutrons. For example: carbon, nitrogen and oxygen all have 8 neutrons.

The atomic numbers of various elements along with electronic configuration and valency is given below

Atomic numbers from 1 to 20 are as follows

## 5. Atomic Number And Mass Number

Rutherford’s Atomic Model

→ In his famous ‘α-ray Scattering Experiment’, Rutherford bombarded α-ray (Helium nucleus 2 H

upon thin gold foil.

• Observations made by Rutherford in his experiment:

(i) Most of α-particles passed through gold foil undeflected.

(ii) Some of the α-particles deflected by foil by small angles.

(iii) One out of every 12000 particles appeared to rebound.

(i) Atom consists of predominantly empty space as most of α-particles passed through gold foil undeflected.

(ii) Atom contains centrally placed positively charged nucleus (carrying positively charged particles), because few α-particles suffered deflected and very few i.e., one in 12000 bounced back.

(iii) Since a minute fraction of α-particles suffered deflections and very few bounced back, this

to conclusion that most of the space an atom is empty and the space occupied by nucleus is negligible compared to this empty space.

→ Size of nucleus was about 10-5 times that of size of atom.

(iv) Whole of the atomic mass concentrated in the nucleus.

• Features of Rutherford proposed model of atom:

(i) There is positively placed nucleus in an atom. Nearly all the mass resides in nucleus (Proton + Neutron).

(ii) Electrons revolves round the nucleus in well defined orbits.

(iii) Size of nucleus is very small compared to the size of atom.

• Drawbacks of Rutherford’s Model (Unstability of Atom)

→ According to Rutherford, electrons revolve round the nucleus in well-defined orbits, but elec

being charged particles will lose their energy and finally will fall into the nucleus.

→ This will make atom highly unstable.

→ This was the major drawback of Rutherford which was unexplained by him.

→ To overcome drawbacks of Rutherford’s Model, Neil Bohr in 1912 proposed modified model of

structure of atom.

→ Only certain special orbits known as discrete orbits of electrons are allowed inside the atom

→ While revolving in discrete orbits, the electrons do not radiate energy.

→ Energy is emitted or absorbed by an atom only when an electron moves from one orbit to another.

## 6. Isotopes

Atomic Number

→ The total number of proton lying in the nucleus of any atom is called the atomic number.

→ An atomic number is the identity of an atom, changing atomic number means changing the atom.

→ Atomic number is denoted by ‘Z’.

→ Atomic number = no. of protons or a neutral atom, no. of protons and electrons are equal.

Mass Number

→ It is the sum of total number of protons and no. of neutrons lying in the nucleus of an atom.

→ It is denoted by ‘A’.

→ Mass number = no. of protons + no. of neutrons

→ Representation of an atom:

zXA

(X= symbol of an element)

Example: Calculate number of protons, electrons and neutrons for 17Cl35 or 17Cl37

Sol. Since Cl is neutral,

No. of electrons = no. of protons = 17

Mass no. of Cl = 35

No. of neutrons = 35 - 17 =18

Distribution Of Electrons In Various Shells

→ The distribution of electrons in various shells is done in accordance to ‘Bohr-Bury Scheme’.

Bohr-Bury Scheme

(i) The filling of electrons in an atom is done in accordance to ‘2n2’, where ‘n’ is the number of shell and ‘2n2 ’ represents the total number of electrons that can be accommodated in that particular shell.

→ Maximum number of electrons that can be filled in particular shell.

If n = 1, i.e., K = shell, 2n2= 2×1 2 = 2 electrons

If n = 2, i.e., L = shell, 2n2 = 2×2 2 = 8 electrons

If n = 3, i.e., M = shell,  2n2 = 2×3 2 = 18 electrons

If n = 4, i.e., N = shell, 2n2 = 2×4 2 = 32 electrons

(ii) The outermost shell can’t hold more than 8 electrons, while second last shell can’t have more than 18 electrons, even though they may have capacity to hold more electrons.

Example: ‘Ca 20 ’, the electron distribution will be :

Ca 20 = 2(K), 8(L), 8(M), 2(N)

→ But Ca 20 = 2, 8, 10 is wrong although ‘M’ shell can contain upto 18 electrons.

(iii) The outermost shell can’t hold more than 2 electrons and the penultimate shell can’t hold more than 8 electrons unless the preceding inner shell (antepenultimate shell) is filled completely obey ‘2n2 ’ rule.

(i) K(19) = 2, 8, 8, 1

(ii) Al (13) = 2, 8, 3

(iii) F (9) = 2, 7

(iv) Ne (10) = 2, 8

(v) Na (11) = 2, 8, 1

Valence Shell and Valence Electrons

→ From Bohr-Bury sequence, we know that maximum number of electrons which can be accommodated in outermost shell is 8.

→ Every element has an urge to have 8 electrons in its outermost shell, in achieving 8 electrons atom can either gain electrons or loose electrons.

→ The number of electrons lost or gained by an element in achieving 8 electrons in its outermost shell will be called its Valency.

→ For elements like H, He, Li, Be and B, these elements lose their outermost electron to achieve 2 electrons in their outermost shell.

→ Isotopes are atoms of same elements having same atomic number and different mass number.

Example: Chlorine has two isotopes of mass numbers 35 and 37 respectively.

17Cl35 , 17Cl37

Uses of isotopes

(i) Uranium isotope is used as fuel in nuclear rector.

(ii) Isotope of cobalt is useful in treatment of cancer.

(iii) An isotope of iodine is used in the treatment of goitre.

Isobars

→ Isobars are the atoms of those elements which have the same mass number but different atomic

numbers are called isobars.

20Ca40 and 18Ar40 have same mass number and different atomic number. 11Na24  and 12Mg24

another examples.