Radionuclide

A radionuclide is an atom with an unstable nucleus, characterized by excess energy available to be imparted either to a newly created radiation particle within the nucleus or via internal conversion. During this process, the radionuclide is said to undergo radioactive decay, resulting in the emission of gamma ray(s) and/or subatomic particles such as alpha or beta particles. These emissions constitute ionizing radiation. Radionuclides occur naturally, or can be produced artificially.

Radionuclides are often referred to by chemists and physicists as radioactive isotopes or radioisotopes. Radioisotopes with suitable half-lives play an important part in a number of technologies (for example, nuclear medicine). Radionuclides can also present both real and perceived dangers to health.

The number of radionuclides is uncertain because the number of very short-lived radionuclides that have yet to be characterized is extremely large and potentially unquantifiable. Even the number of long-lived radionuclides is uncertain (to a lesser degree), because many "stable" nuclides are calculated to have half-lives so long that their decay has not been experimentally measured. The total list of nuclides contains 90 nuclides that are theoretically stable, and 255 total stable nuclides that have not been observed to decay. In addition, there exist about 650 radionuclides that have been experimentally observed to decay, with half-lives longer than 60 minutes (see list of nuclides for this list). Of these, about 339 are known from nature (they have been observed on Earth, and not as a consequence of man-made activities).

Including artificially produced nuclides, more than 3300 nuclides are known (including ~3000 radionuclides), many of which (> ~2400) have decay half-lives shorter than 60 minutes. This list expands as new radionuclides with very short half-lives are characterized.

All elements form a number of radionuclides, although the half lives of many are too short for them to be observed in nature. Even the lightest element, hydrogen, has a well-known radioisotope, tritium. The heaviest elements (heavier than bismuth) exist only as radionuclides. For every chemical element, many radioisotopes that do not occur in nature (due to short half lives or the lack of an ongoing natural production mechanism), have been produced artificially.