A type radioactive decay in which a heavy atomic nucleus splits into two or more lighter nuclei. In comparison to induced fission, there is no inciting particle to trigger the decay, it is a purely probabilistic process. Spontaneous fission is a dominant decay mode for superheavy elements, where the nuclear stability decreases as nuclear mass increases. It thus forms a practical limit to heavy element nucleon number. - Heavier nuclides may be created instantaneously by physical processes, both natural (via the r-process) and artificial, though rapidly decay to more stable nuclides. > [!note] Not Naturally Occurring > > Apart from minor decay branches in primordial radionuclides (residues from the Big Bang nucleosynthesis, cosmic ray spallation, and ancient supernova), spontaneous fission is not observed in nature.