ES0702  Observe an animation of nuclear fission.
  !   Click the image to see the animation.
Jennifer Loomis, TERC

A stable atom of Uranium-235 becomes an unstable (radioactive) atom of Uranium-236 when it absorbs a neutron. The nucleus of an unstable U-236 atom breaks apart spontaneously, releasing a large amount of energy, nuclei of two lighter elements, and three neutrons.

Other U-235 atoms can absorb the neutrons that are released in the reaction, forming more unstable atoms. As increasing numbers of unstable atoms experience fission, more energy and neutrons are released, resulting in a chain reaction of fission.