Discovery of NEUTRON

Atom was supposed to be consisting of only two fundamental particles i.e. electron and proton. The mass of atom was considered due to the proton only because electron has negligible mass. Each proton has mass of 1.67×10–24g which is taken as one unit mass. Rutherford found that except for Hydrogen atom, the atomic masses of no other atom can be explained on the basis of protons and electrons only. For example, the Helium atom has two protons and two electrons only. Therefore its atomic mass must be twice the mass of proton i.e. two units, but the actual mass of Helium is 4 units. So, Rutherford predicted the presence of some neutral particle in the nucleus which has mass equal to the mass of proton. Thus, the nucleus of Helium must have two such neutral particles.
In 1932, Sir James Chadwick discovered the third subatomic particle. He bombarded the beryllium foil with alpha particles and found that a new particle was ejected. The mass of such particle was found to be almost equal to that of proton and has no charge. This particle was named as Neutron (n).