The third fundamental particle is the neutron, discovered in 1932 by James Chadwick (1891-1974) by bombarding a sheet of beryllium with alpha particles, he observed the emission by the metal of very high energy radiation, similar to gamma rays. Later studies showed that said radiation was made up of neutral particles (they do not respond to electric fields) with a mass slightly greater than that of protons.

The discovery of the neutron allowed us to understand the reason why the helium atom has a mass 4 times greater than that of hydrogen, containing only two protons. The explanation lies in the existence of 2 neutrons in its nucleus.