Discovery of Electron

Chapter:- 4

Structure of The Atom

Charged Particles in Matter

Some charged particles are present within the atom or atom is made up of some charged Two such particles are electrons and protons.

Discovery of Electrons

The electron, was identified by JJ Thomson, when he performed cathode ray experiment using a discharge tube. In this experiment, he found a beam of negatively charged particles, called the cathode rays, as they were originated from the cathode.

The Characteristics Exhibited by Cathode Rays are

  • These rays travel in a straight line and rotate a pin wheel placed in their path. This shows that these consist of material particles possessing high kinetic energy.
  • In the presence of  electric field, these get deflected towards the positive electrode. This shows that these rays are made up of negatively charged particles.
  • Millikan found the charge on electron (1.6 x 10–19C).
  • Mass of electron is (9.1 x 10–31kg).

Discovery of Proton

E Goldstein in 1886, discovered the presence of new radiations known as canal rays or anode rays.  These are the positively charged rays which are seen moving from the anode towards cathode in a specially designed discharge tube when a high voltage is applied across the electrodes. Porous cathode is used to provide the path for passing anode rays. It led to the discovery of another sub-atomic particle, the proton (1.602 x 10–19C).

Some Characteristics Features of Anode Rays are

  • These rays always travel in a straight line.
  • Anode rays get deflected by electric and magnetic fields in a direction opposite to the cathode rays.
  • These consist of positively charged particles, known as protons.
  • Proton had a charge, equal in magnitude but opposite in sign to that of electron. Its mass was 1840 times. i.e., approximately 2000 times, as that of the electron.
  • The e/m values for the particles are not the same for all the gases enclosed in the discharge tube.
  • The nature of anode rays depends upon the nature of gas enclosed in the discharge tube.


In general, an electron is represented as e and a proton as p+. The mass of a proton is taken as one unit and its charge as plus one (+1), where as the mass of an electron is considered to be negligible and its charge is minus one (–1). It seemed likely that an atom was composed of protons and electrons, mutually balancing their charge.

The Structure of an Atom

According to Dalton’s atomic theory, atom was indivisible and inderstuctible. The discovery of two fundamental particles inside the atom, led to the failure of this aspect of Dalton’s Theory. To know the arrangement of electrons and protons within an atom, many scientists proposed various atomic models.

Thomson’s Model of an Atom

Thomson's model of an atom was similar to watermelon. The electrons in a sphere of positive charge, were like seeds in a spherical watermelon. In which, the positive charge in an atom is spread all over like the red edible part, while the electrons studded in the positively charged sphere, like the seeds in the watermelon.

Following are The Postulates of This Model

  • Electrons are embedded in the sphere of positive charge.
  • The negative and positive charges balance each other. Therefore, atom as a whole is neutral.
  • Mass of an atom is due to the electrons.

Limitations of Thomson's Model of an Atom

  • JJ Thomson's model could not explain the experimental results of other scientists, as there is no nucleus in the atomic model proposed by Thomson.
  • It does not have any experimental evidence in its support.

Rutherford's Model of an Atom

Ernest Rutherford bombarded thin sheets of gold foil with fast moving α-particles (These are doubly charged helium ions having a mass of 4 u). He selected a gold foil because he wanted a layer as thin as possible. Thin gold foil was about 1000 atoms thick.

The Following Observations Were Made By Rutherford

  • Most of the fast moving α -particles passed straight through the gold foil.
  • Some of the α -particles were deflected by the foil by small angles.
  • Very few α -particles (one out of 12000) appeared to rebound.

On the basis of his experiment, Rutherford put forward the nuclear model of an atom, having the following features

  • There is a positively charged, highly densed centre in an atom, called the nucleus. Nearly, the whole mass of the atom resides in the nucleus.
  • The electrons revolve around the nucleus.
  • The size of the nucleus (10 -I5 m) is very small as compared to the size of the atom (10-10 m).

Limitations of Rutherford's Model of an Atom

  • Any charged particle when accelerated is expected to radiate energy.  Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable.
  • Rutherford's model could not explain the distribution of electrons in the extra nuclear portion of the atom.

Bohr's Model of an Atom

Bohr's Model of an Atom

To overcome the objections raised against Rutherford's model of the atom, Neils Bohr put forward the following postulates about the model of an atom.

(i):- Atom consists of positively charged nucleus around which electrons revolve in discrete orbits i.e., electrons revolve in certain permissible orbits and not just in any orbit.

(ii):- Each of these orbits are associated with certain value of energy. Hence, these orbits are called energy shells or energy levels.

(iii):- Starting from nucleus, energy levels (orbits) are represented by numbers (1, 2, 3, 4, etc) or by alphabets (K, L, M, N, etc).

(iv):- Normally, the electrons present in first energy level (E1) have lowest energy. k Energies increase going towards outer energy levels.

(v):- Energy of an electron remains same as long as it remains in discrete orbit and it does not radiate energy while revolving.

(vi):- When energy is supplied to an electron, it can go to higher energy levels. While an electron falls to lower energy level, it radiate energy.

Neutrons (n)

Neutrons are another sub-atomic particles, discovered by J Chadwick in 1932. It is represented by n. Neutrons are electrically neutral particles and are as heavy as protons.

(i.e., their mass is 1.67493x10–27 kg). Neutrons are present in the nucleus of all atoms except hydrogen. The mass of the atom is given by the sum of the masses of protons and neutrons present in the nucleus.

Distribution of Electrons in Different Orbits (Shells)

For writing the number of electrons in different energy levels, there are some rules.

  • The maximum number of electrons present in a shell is given by the formula    2n 2, where n is the orbit number or energy level, 1, 2, 3…….     

Therefore, the maximum number of electrons in different shells are as follows

First orbit or K-shell = 2 x (1)2 = 2

Second orbit or L-shell = 2 x (2)2 = 8

Third orbit or M-shell = 2 x (3)2 =18

Fourth orbit or N-shell = 2 x (4)2 = 32 and so on.

  • The maximum number of electrons that can be accommodated in the outermost orbit is 8.
  • Electrons are not accommodated in a given shell, unless the inner shells are filled.


The electrons present in the outermost shell of an atom are known as the valence electrons. The combining capacity of the atom of an element with the atom(s) of other element(s) in order to complete its octet is known as valency.

Atomic Number

It is defined as the number of protons present in the nucleus of an atom. It is nearly denoted by Z.

For Example:-

Mass Number

It is defined as the sum of number of protons and neutrons present in the nucleus of an atom. Mass number is denoted by A.

Mass number (A) = Number of protons + Number of  neutrons

Number of neutrons = Mass number – Atomic number

  Atomic number = Number of protons

In the notation for an atom, the atomic number, mass number and symbol of the element are to be written as



Atomic Weight

Mass of an individual atom is called atomic mass or atomic weight. It is nearly equal to the mass number.

For Example:-

(i):-  Carbon, atomic number (Z) = 6,

Mass number (A) =12,

Atomic mass =12.01 u

(ii):- Oxygen, atomic number (Z) = 8,

mass number (A) = 16, atomic mass = 15.99

Different Atomic Species; Isotopes

These are defined as the atoms of the same element, having the same atomic number but different mass numbers. For Example:- there are 3 isotopes of hydrogen atom, namely protium ,deuterium   and tritium        

For Example:- The two isotopic forms of chlorine atom with masses 35 u and 37 u occur in the ratio of 3 : 1.

Therefore, the average atomic mass of chlorine atom, can be calculated as

Applications of Isotopes

  • An isotope of uranium (U-235) is used as a fuel for the production of electricity in nuclear reactors.
  • U-238 is used to determine the age of very old rocks and even the age of the earth.
  • An isotope of cobalt (Co-60) is used in the treatment of cancer.
  • An isotope of carbon (C-14) is used to determine the age of old specimens of wood or old bones of living organisms.
  • An isotope of iodine (I-131) is used in the treatment of goitre.
  • P-32 is used in agricultural research.
  • Na-24 is used to detect blood clots.


Atoms of different elements with different atomic numbers, but same mass number, are known as isobars.


Atoms of different elements having the same number of neutrons are called isotones.