Adding to or removing neutrons from the nucleus of a stable atom results in an unstable isotope. But any unstable isotope will try to regain its equilibrium, throwing off parts of its nucleus in a quest for stability—producing either another isotope or sometimes a different element altogether. For example, plutonium 239 sheds two protons and two neutrons from its nucleus to become uranium 235. This dynamic process of nuclear decay is radioactivity; the energy it releases, as atoms shed neutrons in the form of waves or particles, is radiation.
