Discovery of Neutron

Moseley , in 1913 , performed experiments to determine the exact quantity of charge present on the nucleus. The number of units of positive charge present on the nucleus was called the atomic number of the element. Since each unit of positive charge corresponds to one proton, therefore , atomic number was equal to the number of protons present in the nucleus. Further , since each proton has one unit mass on the atomic mass scale , therefore , the mass of the atom was calculated from the number of protons present in the nucleus (remembering that electrons have negligible mass).
Chadwick , in 1932 , performed some scattering experiments in which he bombarded some light elements like beryllium and boron with fast moving alpha - particles. He found that some new particles were emitted which carried no charge I.e. were neutral but had a mass nearly equal to that of proton. This particle was termed 'Neutron'.

Hence , the neutron may be defined as :- A neutron may be defined as that fundamental particle which carries no charge but has a mass nearly equal to that of hydrogen atom or proton.