A radionuclide is an atom with an unstable nucleus, which is a nucleus characterized by excess energy available to be imparted either to a newly created radiation particle within the nucleus or to an atomic electron. The radionuclide, in this process, undergoes radioactive decay, and emits gamma ray(s) and/or subatomic particles. These particles constitute ionizing radiation. Radionuclides occur naturally, and can also be artificially produced.

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

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 constructive technologies (for example, nuclear medicine). Radionuclides can also present both real and perceived dangers to health.

Source: Wikipedia

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