Chapter 5: Periodic Classification of Elements

Chapter 5

Periodic Classification of Elements

Introduction

At present, 118 elements are known to us.
All these have different properties. Out of these 118, only 98 are naturally occurring.
These elements have different characteristic properties, so it is very difficult to study these elements individually.
Dobereiner’s Triads (1817):

He identified some triads (groups having three elements).
Dobereiner showed that when the three elements in a triad were written in the order of increasing atomic masses; the atomic mass of the middle element was the average of the atomic masses of the other three elements. For example, Dobereiner could identify only three triads from the elements known at that time. Hence, this system of classification into triads was not found to be useful.

Newland’s Law of Octaves (1866):

• He arranged the known elements in the order of increasing atomic masses.
• He found that every eighth element had properties similar to that of the first like the musical note. It is known as Newlands law of octaves.

 Limitations

• Law of octaves was applicable only upto calcium.
• Discovery of Noble gases disturbed the octaves.
• Octaves worked well only for lighter elements.
• Properties of new elements could not fit in it.

Periodic classification of elements

Sample paper

1. What is the mendeleev periodic law?
2. What is modern periodic law?
3. What is the name given to the horizontal rows and vertical column
4. What are the drawbacks of the mendeleev periodic law
5. How many groups and periods are found in the mendeleev periodic law
6. Why noble gases are not present in the mendeleev periodic table
7. How many groups and periods are found in the long form of table
8. What is ionization enthalpy why the second ionization enthalpy of alkali metals is greater than the first ionization enthalpy
9. Why the atomic radius of chlorine is less than atomic radius of sodium
10. What is electronegativity how does electronegativity variates in periodic table from moving top to bottom
11. Why hydrogen is placed at the top of the periodic table
12. Who discover the atomic number
13. What are diagonal elements
14. What is electron affinity how does it changes when we move from left to right in the periodic table
15. Why noble gases do not react with any other elements

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Mendeleev’s Periodic Table

Mendeleev’s Periodic Law:

Physical and chemical properties of elements are the periodic function of their atomic masses.
Mendeleev’s periodic table is based on the chemical properties of elements.
Mendeleev’s periodic table contains vertical columns called ‘groups’ and horizontal rows called ‘periods’.
Achievements of Mendeleev’s Periodic Table: 63 elements were known at the time of classification.

• Elements with similar properties could be grouped together.
• Mendeleev left some gaps in his periodic table. Mendeleev boldly predicted the existence of some elements that had not been discovered at that time.
• Noble gases discovered, could be placed without disturbing the existing order.

Remember
Scandium, gallium and germa-nium have properties similar to Eka-boron, Eka-aluminium and Eka-silicon respectively.

Limitations of Mendeleev’s Classification

• The position of hydrogen in the table was not certain because it could be placed in the group of alkali metals as well as in halogens.
• Isotopes of elements were placed in the same position in the table though according to their atomic masses, they should have been placed in different positions.
• Certain elements of higher atomic mass proceed those with lower atomic mass. For example, tellurium (atomic mass 127.6) precedes iodine (atomic mass 126.9). Iodine was placed after tellurium though it had lower atomic mass because Iodine had properties similar to bromine and not selenium.

The Modern Periodic Table:

Periodicity. The properties which reoccur after a regular intervals in a periodic table are called periodic properties and the phenomenon is called periodicity of element.
Henry Moseley (1913) exhibited that the atomic number of an element is a more fundamental property than its atomic mass. Mendeleev’s periodic law was modified and atomic number was adopted as the basis of the modern periodic table.

The Modern Periodic Law states that:
The physical and chemical properties of the elements are the periodic function of their atomic numbers.
It means that if the elements are arranged in order of increasing atomic numbers, the elements with similar properties recur after regular intervals.
Many new forms of periodic table have been proposed in recent times with modern periodic law as the guiding principle, but the general plan of the table remains the same as proposed by Mendeleev. The most commonly known periodic table is the Long form of the periodic table.Few Points to remember:

• Modern periodic table contains 18 vertical columns known as groups and 7 horizontal rows known as periods.
• Elements in a group have the same number of valence electrons.
• Number of the shells increases as we go down the group.
• Elements in a period have same number of shells.
• Number of elements placed in a particular period depends upon the fact that how electrons are filled into various shells.
• Maximum number of electrons that can be accommodated in a shell depends on the formula 2n² where n is the number of the given shell. For example, in K shell the number of electrons is 2 x (1)² = 2 in the first period, L shell – 2 x (2)² = 8 elements in the second period.
• Position of the element in the periodic table tells about its reactivity.

Trends in the Modern Periodic Table
Valency: Number of valence electrons present in the outermost shells. Valency remains the same down a group but changes across a period.
Atomic Size: Atomic size refers to radius of an atom.
Atomic size or radius decreases in moving from left to right along a period due to increase in nuclear charge. Atomic size increases down the group because new shells are being added as we go down the group.
Metallic Character: Metallic character means the tendency of an atom to lose electrons.
Metallic character decreases across a period because the effective nuclear charge increases, that means the tendency to lose electrons decreases. Metals are electropositive as they tend to lose electrons while forming bonds.