Radioactive conversion emitting an alpha particle. During alpha decay the atomic number is reduced by two units and the mass number by four units. For example, alpha decay generates Rn-222 with the atomic number 86 and the mass number 222 from Ra-226 with the atomic number 88 and the mass number 226.

Diagram alpha decay

Alpha decay; the figure shows the decay of radium-226 into radon-222 emitting helium with 4 nucleons (2 protons and 2 neutrons = 1alpha particle)