Alpha decay or α - decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or ' decays ' into a different atomic nucleus, with a mass number that is reduced by four and an atomic number that is reduced by two. An alpha particle is identical to the nucleus of a helium - 4 atom, which consists of two protons and two neutrons. It has a charge of 6981320435297400000 ♠ + 2 e and a mass of 7000400000000000000 ♠ 4 u. For example, uranium - 238 decays to form thorium - 234. Alpha particles have a charge 6981320435297400000 ♠ + 2 e, but as a nuclear equation describes a nuclear reaction without considering the electrons -- a convention that does not imply that the nuclei necessarily occur in neutral atoms -- the charge is not usually shown.
if uranium-238 undergoes alpha decay the daughter nucleus will be
thorium - 234