Nuclear fission is an atomic reaction in which the nucleus of a heavy element splits. It can occur spontaneously or when the nucleus captures a neutron. During fission reactions the parent nucleus typically splits into several smaller fragments and two or three neutrons.
The daughter nuclei that are emitted possess approximately half the mass of the parent nuclei. The sum of their masses is equal to but slightly less than the original mass. This 'missing' mass (about 0.1 percent of the original mass) has been converted into energy according to Einstein's equation..
History of Nuclear Fission
Ernest Rutherford was said to be the first person split the nucleus, in 1917 at Manchester University where he bombarded nitrogen with alpha particles and observed a proton emitted with energy higher than the alpha particle. The nitrogen was then converted into oxygen by nuclear fission.