Discovery of Neutron:
In 1932, Sir James Chadwick discovered the third subatomic particle. He directed a stream of alpha particles (2He4) at a beryllium target. He found that a new particle was ejected. It has almost the same mass (1.674 x 1014 g) as a proton and has no charge.
He named it neutron (n). The assigned relative mass of a neutron is approximately one atomic mass unit (amu). Thus,
“A neutron is a subatomic particle which has a mass almost equal to that of a proton and has no charge”.
The reaction which occurred in Chadwick’s experiment is an example of artificial transmutation where an atom of Beryllium is converted to a carbon atom through the nuclear reaction.
2He4 + 4Be9 ——— 6C12 + 0n1
Thus, an atom consists of three fundamental particles; electron, proton and neutron. This means that the nucleus consists of protons and neutrons and these are collectively known as nucleons. Since the electrons are of negligible mass, the entire mass of the atom is due to the nucleus i.e. nucleons. The sum of the neutrons and protons is known as mass number.