Transcript: The decay of a massive atomic nucleus with the release of particles or energy or the splitting of a massive nucleus into two or more pieces is called fission. In fission the sum of the fragments is less than the mass of the original nucleus. The excess is released as energy according to E = mc2. Fission is a highly efficient energy source. When a single atom of uranium 235 decays it releases three times ten to the minus eleven Joules of energy. Not much for a single atom, but a fistful is sufficient to power a city. Contrast this with a chemical energy source such as coal or any fossil fuel. In this case a single atom of carbon combining with oxygen to carbon dioxide releases only six times ten to the minus nineteen Joules, fifty million times less energy.