Nuclear fission
Definition
The splitting of a heavy atomic nucleus into two lighter nuclei, releasing a large burst of energy. This is the reaction that powers nuclear power plants.
Unit: electronvolts (eV) or joules (J) for the energy released
Technical definition
A nuclear reaction in which a heavy nucleus (such as uranium-235) absorbs a neutron and splits into two lighter nuclei plus additional neutrons and gamma radiation. The products weigh slightly less than the starting nucleus; the missing mass becomes energy via E = mc^2.
Example
When one uranium-235 nucleus fissions, it releases roughly 200 million electronvolts — millions of times more than a typical chemical reaction. In a reactor, the extra neutrons trigger further fissions in a controlled chain reaction.
Fission (splitting heavy nuclei) and fusion (joining light nuclei) are opposites — students frequently swap them. Remember: fission = 'fissure,' a crack that splits something apart. Fission also happens controllably inside every nuclear power plant, not only in weapons.