Many heavier elements with smaller binding energies per nucleon can decompose into more stable elements that have intermediate-mass numbers and larger binding energies per nucleon—that is, mass, numbers, and binding energies per nucleon that are closer to the “peak” of the binding energy graph near 56. Sometimes neutrons are also produced. This decomposition is called fission, the breaking of a large nucleus into smaller pieces. The breaking is rather random with the formation of a large number of different products. Fission usually does not occur naturally but is induced by bombardment with neutrons.

The first reported nuclear fission occurred in 1939 when three German scientists, Lise Meitner, Otto Hahn, and Fritz Strassman, bombarded uranium-235 atoms with slow-moving neutrons that split the U-238 nuclei into smaller fragments that consisted of several neutrons and elements near the middle of the periodic table. Since then, fission observed in many other isotopes, including most actinide isotopes that have an odd number of neutrons.