The discovery of proton brought the concept that atomic mass should be equal to the total mass of proton of the atoms since the mass of electron negligible i.e., mass of an electron is very small with respect to mass of a proton. However it has been found that in all atoms, expect hydrogen, the mass of an atom is more than total mass of protons present in the atom.
To solve this problem, Chadwick performed an experiment, in 1932, (who was awarded by Nobel Prize for this work) by bombarding alpha particles on thin sheet of beryllium element. He observed highly penetrating rays consisting of streams of neutral particles were found to have a mass of 1.675*10^-24gm which is nearly equal to the mass of hydrogen atom. These neutral particles were named as neutrons. The fundamental particle which carries no charge but has a mass nearly equal to that of proton is known as neutrons.
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