Discovery of Neutron

Rutherford's model was unable to explain the relationship between the atomic mass and the atomic number (the number of protons). According to the Rutherford’s model, the mass of helium atom (containing 2 protons) should be double that of a hydrogen atom (with only one proton).

However, the actual ratio of the masses of helium atom to hydrogen atom is 4:1. It was suggested that there must be one more type of subatomic particle present in the nucleus which may be neutral but have mass.

Such a particle was discovered by James Chadwick in 1932. This was found to be electrically neutral and was named neutron. Neutrons are present in the nucleus of all atoms, except hydrogen.

A neutron is represented as n and is found to have a mass slightly higher than that of a proton. Thus, if the helium atom contained 2 protons and 2 neutrons in the nucleus, the mass ratio of helium to hydrogen (4:1) could be explained.

Characteristics of Fundamental Subatomic Particles