Although Rutherford’s model of the atom could explain the electrical neutrality and the results of scattering experiment but a major problem regarding the atomic masses remained unsolved.
The mass of helium atom (which contains 2 protons) should be double than that of a hydrogen atom (which contains only one proton). [The electron being very light weight particle as compared to that of a proton, its contribution to the atomic mass can be ignored]. Actual ratio of helium and hydrogen masses is 4:1. Rutherford and others, thus, suggested that there must be one more type of subatomic particle present in the nucleus which may be neutral but must have mass. Later in 1932, James Chadwick showed the existence of this third type of subatomic particle. This was named as neutron. The neutron was found to have a mass slightly higher than that of a proton electrically neutral. Thus, if the helium atom contained 2 protons and 2 neutrons in the nucleus, its mass ratio to hydrogen as 4:1 could be explained. The characteristics of these three particles, called as fundamental particles are given in Table 3.1.

James Chadwick (1891-1972) was a British physicist. He received the Nobel prize in 1935 for showing the existence of neutron in the nucleus of an atom.

Particle	Symbol	Mass (kg)	Charge	Coulomb (C) in multiple units
Electron	e	9.10939*10-31	-1.6022*10-19	-1
Proton	p	1.67262*10-27	+1.6022*10-19	+1
Neutron	n	1.67493*10-27	0	0