Radioactivity

Radioactivity is the property of energy or particle release as the result of changes in the atomic nuclei of elements, from nuclear fission or nuclear fusion. Energy release is in the electromagnetic spectrum, in the gamma radiation range. Particles released may be neutrons, alpha particles, or beta particles.

Not all elements exhibit radioactivity, and, for a specific element, there are stable (i.e., nonradioactive) and radioactive isotopes. Nuclei are composed of one or more protons and, with one exception, one or more neutrons. The number of protons defines the atomic number, and the combination of protons and neutrons defines the atomic mass. A radioactive isotope lacks some number of neutrons needed to stabilize the nucleus. For example, uranium, with an atomic number of 92, has the almost completely stable isotope ²³⁸U, with a half-life of 4.46 × 10⁹ years. Its isotope ²³⁵ is still quite stable with a half-life of 7.13 × 10⁸ years, but will fission when its nucleus is struck by a neutron.

It is impossible to predict when a given nucleus will emit particles or energy, but, with a sufficiently large number, it is possible to speak of the half-life, or time within which half the nuclei will undergo radioactive decay.