Elements that result from the fission of an atomic nucleus. When a neutron hits an atomic nucleus, it splits it into two or more nuclei. These are called fission products. Very often, these are unstable isotopes that undergo radioactive decay to become more stable. Examples of fission products are krypton and barium, which are produced by the fission of uranium ²³⁵U. In addition to the nuclei of the atoms, several neutrons are released.