Mass-energy equivalence: E=MC2:
A nucleus of uranium 235 which splits after having absorbed a neutron is slightly heavier than the two fission products and secondary neutrons put together. This mass difference is tiny (0.2% of the total) but highly significant. When multiplied by the speed of light squared (in order to obtain its equivalent energy) the total amounts to almost 200 million electronvolts, a huge number compared to the few electronvolts of normal chemical reactions.
Jean-Marc Faday (CNRS/Images-M├ędia)