Although Dalton had postulated that atoms were indivisible particles, experiments conducted around the beginning 21^st century showed that atoms themselves consist of particles. These experiments showed that an atom consists of two kinds of particles: a nucleus, the atom’s central core, which is positively charged and contains most of the atom’s mass, and one or more electrons.

Discovery of the electrons

Do you think that there is a similarity between Cathode rays and electrons?

In 1879, the English scientist William Crookes (1832-1919) experimented with gas discharge tubes.

These radiations are called cathode rays. Fig 1a, shows emission of cathode rays in a discharge tube. At this stage the glass walls of the discharge tube opposite to the cathode starts glowing with a faint greenish light.

Cathode rays normally travel in straight lines, but are deflected when a magnet is brought nearby.

Thomson concluded that these negatively charged particles were constituents of every kind of atom. We now call these particles electrons, a term that had been coined by the Irish Physicist George Stoney in 1891 to describe the smallest unit of electric charge..

Thomson’s model of an atom

• According to Thomson,
  (i) An atom consists of a positively charged sphere and the electrons are embedded in it.
  (ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.
• The first model of an atom to be put forward and taken into consideration.
• He proposed a model of the atom be similar to that of a Christmas pudding/watermelon.
• The red edible part of the watermelon is compared with the positive charge in the atom.

Limitations of Thomson’s atomic model:

• It failed to explain the stability of an atom because his model of atom failed to explain how a positive charge holds the negatively charged electrons in an atom. Therefore, This theory also failed to account for the position of the nucleus in an atom.
• No experimental evidence in its support.

Discovery of nucleus

In 1911, the New Zealand chemist and physicist, Ernest Rutherford, who had studied with J.J. Thomson decided to use α-particles to probe the structure of atoms.

Rutherford’s model of an atom

Rutherford concluded the model of the atom from the α-particle scattering experiment as:

1. There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
2. The electrons revolve around the nucleus in well-defined orbits.
3. The size of the nucleus is very small as compared to the size of the atom.

Drawbacks of Rutherford’s model

Rutherford explained that the electrons in an atom revolve around the nucleus in well-defined orbits. Particles in a circular orbit would experience acceleration.

Discovery of neutron

In the 1920s and early 1930s, alpha-particles were used as projectiles to bombard a variety of materials. Bombardment of beryllium atoms produced a strange, highly penetrating form of radiation. In 1932, James Chadwick (1891-1972) showed that this radiation was best explained as a beam of neutral particles.

Bohr’s model of an atom

Bohr suggested the following postulates to overcome the objections raised against Rutherford's model:

• Electrons revolve around the nucleus in stable orbits without emission of radiant energy. Each orbit has a definite energy and is called an energy shell or energy level.
• An orbit or energy level is designated as K, L, M, N shells. When the electron is in the lowest energy level, it is said to be in the ground state.
• An electron emits or absorbs energy when it jumps from one orbit or energy level to another.
• When it jumps from a higher energy level to lower energy level, it emits energy while it absorbs energy when it jumps from lower energy level to higher energy level

Bohr’s model gives an elaborative explanation on the structure of an atom and overcomes the objections faced by all the other models on the structure of an atom.