Atom Thomson Model:

Experiments by J.J. Thomson showed the existence of electrons. Radioactivity proved the fact that an atom consists of positively and negatively charged particles. As the atom is electrically neutral, the positive charge must be equal to the negative charge. The question arose as to have the electrons are arranged in the atom. Thomson suggested that the atom was spherical in shape. The whole mass of the atom was evenly disturbed and the positive charge was distributed all over the mass. The electrons with negative charge were embedded within the atom. The whole positive charge of the atom was equal to the total charge of the electrons. The atoms was like a plum pudding. The electrons oscillate or vibrate about their mean positions. The Thomson model could be successfully applied in simple gas laws and in simple physical and chemical problems.

The characteristics of Thomson model were:

1. It explained the emission of electrons by heating as thermions and in photo-electric effect.

2. It provides a mechanism for the emission of the electromagnetic waves. The electro-magnetic waves were supposed to be emitted by oscillating bound electrons in the atom.

3. It provided an elastic hard model required for the kinetic theory of gases.

4. The dimensions of the atom were assumed as given by the mean free path and Wan der Waals considerations.

5. It explained the existence of positively charged and negatively charged ions. A gain in electron by the neutral atom produces a negative ion and the loss of an electron by the neutral atom produces a positive ion.

Failure: The scattering of α-particles by the heavy atoms studied by Rutherford led him to the idea that the positive charge could not be disturbed evenly in the while mass of the atom.