Nuclear fission is the process of bombarding high-density, high-molecular weight atoms with neutrons in order to cause them to split, thereby releasing two lower-molecular weight atoms and a tremendous amount of energy. These kinds of reactions usually involve isotopes, or atoms that contain a different number of neutrons than protons in the nucleus. Thus, the proper chemical equation will usually represent a neutron colliding with a high-molecular weight molecule as reactants and the smaller-molecular weight molecules as products, as well as the energy release.

Additionally, the breakup of a large atom not only produces atoms of smaller molecular weight, but also more free neutrons. These neutrons, once they are released from the nucleus of the larger-molecular weight molecule, are then free to collide with other molecules, initiating what is called a nuclear chain reaction.

The most common type of fission reaction is when a neutron collides with a molecule of Uranium-235. The first product of this reaction is an extremely short-lived molecule of Uranium-236 (which has just absorbed the incoming neutron). This molecule, however, is extremely unstable and quickly breaks down into three separate products: Barium-144, Krypton-86, and three neutrons.

The reaction also produces gamma-ray radiation and energy. The energy release in Joules for nuclear reactions can often be looked up in a chemical table.