Alpha decay

In alpha decay an atomic nucleus ejects an alpha particle and transforms into a nucleus with [mass number]? 4 less and atomic number 2 less. For example

                     23892U -->23490Th + 42He

More usually written as

                    238U-->234Th + α

Note that an alpha particle is a helium nucleus, and that both mass number and atomic number are conserved. Essentially alpha decay can be thought of as nuclear fission where the parent nucleus splits into two daughter nuclei.