Fission occurs when a larger nucleus, or parent isotope, splits into two or more smaller nuclei, or daughter isotopes. One way in which fission occurs is when a large nucleus is struck by a slow moving neutron. If this fission reaction causes the release of additional neutrons, which will cause further fission reactions in the sample, the reaction is called a chain reaction. Chain reactions from a starting element to an end product may follow more than one series of reactions, or decay modes.
Fission reactions conserve the atomic mass. Therefore the total number of protons and neutrons before and after the reaction must be the same.
