<Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> 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 an atom 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 + 2e and a mass of 4u . For example, uranium - 238 decays to form thorium - 234 . Alpha particles have a charge + 2, 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 . </P> <P> Alpha decay typically occurs in the heaviest nuclides . Theoretically, it can occur only in nuclei somewhat heavier than nickel (element 28), where the overall binding energy per nucleon is no longer a minimum and the nuclides are therefore unstable toward spontaneous fission - type processes . In practice, this mode of decay has only been observed in nuclides considerably heavier than nickel, with the lightest known alpha emitters being the lightest isotopes (mass numbers 106--110) of tellurium (element 52). Exceptionally, however, beryllium - 8 decays to two alpha particles . </P>

An alpha particle has a mass number of