What is the splitting of atomic nuclei called?

The splitting of atomic nuclei is called nuclear fission. In fission, a heavy atomic nucleus divides into two or more smaller nuclei, usually releasing neutrons, radiation, and a large amount of energy.

The process was identified in uranium experiments in 1938 by Otto Hahn and Fritz Strassmann, while Lise Meitner and Otto Frisch explained what had happened. Frisch coined the term “fission,” borrowing it from the biological division of cells. Their work showed that a uranium nucleus could split roughly in half rather than merely ejecting a small particle.

Fission releases energy because the resulting nuclei are more tightly bound than the original heavy nucleus. The emitted neutrons can trigger additional fissions, creating a chain reaction. Controlled chain reactions generate heat in nuclear reactors; uncontrolled rapid chain reactions were used in nuclear weapons.

Fission is often confused with nuclear fusion. Fission splits heavy nuclei such as uranium-235, whereas fusion joins light nuclei, such as hydrogen isotopes, and powers the Sun.