Example of the historical decay of radium-226
A radium nucleus is a massive nucleus of 226 nucleons, including 88 protons and 138 neutrons. It decays by emitting an alpha particle composed of two protons and two neutrons. The radium nucleus turns into radon-222 nucleus, itself radioactive, containing two protons and two neutrons less. The disintegration releases 4.6 million electronvolts of energy. The alpha particle carries the 222/226ths of this available energy and the radon 4/226ths. The process is very slow : on average, it is necessary to wait 2300 years for the disintegration.