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.