How was neutron discovered? Describe briefly.

It was Rutherford who showed that almost the entire mass of an atom is concentrated in the nucleus. However, he could not explain the total known mass of the atom in terms of the number of protons in the nucleus. For example, helium nucleus has a charge of 2+ which is furnished by 2 protons, also contributes approximately 2 units of mass. As the mass of helium atom is 4 a.m.u., therefore 2 units of mass are still to be accounted for.

Rutherford postulated the existence of a neutral particle to account for the missing mass. Chadwick in 1932, actually discovered this neutral particle by bombardment of beryllium or boron by alpha particles. It has almost exactly the same mass as of a proton but carried no electric charge. These neutral particles were called neutrons.
Thus, a neutron is a neutral particle having a mass (1.675 x 10^-27 kg) which is almost exactly the same as that of a proton. On the a.m.u. scale, the mass of the neutron is 1.00867.