Alpha decay is a form of radioactive decay in which an atomic nucleus ejects an alpha particle and transforms into a nucleus with mass number 4 less and atomic number 2 less. For example:

although this is usually written as:

Note that an alpha particle is a helium nucleus, and that both mass number and atomic number are conserved. Alpha decay can essentially be thought of as nuclear fission where the parent nucleus splits into two daughter nuclei. Alpha decay is fundamentally a quantum tunneling process. In some radioactive substances when beta decay occurs along with alpha decay, atomic helium is formed.

Most alpha particles are emitted with approximately 5 MeV of energy, for 98% as kinetic energy, i.e. 120 MJ/kg, hence a speed of 15 km/s.