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Discovery of the Neutron


We have seen that mass of atom is mainly concentrated in the nucleus. The nucleus contains not only protons but also another type of particles called neutrons. The existence of neutrons in the nucleus was first predicted by Rutherford in 1920 to account for the difference in the mass of the atom and the total mass of protons. Neutrons were discovered experimentally by James Chadwick in 1932. He bombarded a thin foil of beryllium with fast moving a-particles and observed that highly penetrating rays consisting of neutral particles were produced.

These neutral particles were found to have mass 1 .675 x 10-27 kg and were named neutrons. In general, a neutron is represented as ‘n’.

A neutron is a subatomic particle carrying no charge and having mass 1.675 x 10-27 kg which is almost equal to that of a hydrogen atom.

The mass of the atom is largely due to protons and neutrons in the nucleus of the atom. Atoms of all elements except ordinary hydrogen contain one or more neutrons in their nuclei.

The relative masses and relative charges of these three subatomic particles are summarized in Table.