Radionuclide

A radionuclide is an atom with an unstable nucleus, which is a nucleus characterized by excess energy which is available to be imparted either to a newly-created radiation particle within the nucleus, or else to an atomic electron. The radionuclide, in this process, undergoes radioactive decay, and emits a gamma ray(s) and/or subatomic particles. These particles constitute ionizing radiation. Radionuclides may occur naturally, but can also be artificially produced.
The number of radionuclides is uncertain. Even the number of long lived radionuclides is uncertain, because many "stable" nuclides are calculated to have half lives so long that their decay has not been experimentally measured. The list of nuclides contains 90 nuclides that are theoretically stable, and 257 total nuclides that have not been measured to decay (but of which 257-90 = ~ 167 have never been observed to decay). Thus, there are about 905 - 257 = 648 radionuclides that have been experimentally observed to decay with half lives longer than 60 minutes. More than 3000 radionuclides are known, including those with decay half lives shorter than 60 minutes.
Radionuclides are often referred to by chemists and physicists as radioactive isotopes or radioisotopes. They play an important part in the technologies that are used in a number of constructive technologies (for example, nuclear medicine). However, radionuclides can also present both real and perceived dangers to health.

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