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An atom with an unstable nucleus that undergoes radioactive decay. Also referred to as a radioactive nuclide, radioisotope, or radioactive isotope. Matter containing radionuclides is considered to be radioactive. Radionuclides are commonly found in nature or can be made artificially for scientific or medical purposes. Typical examples of radionuclides are uranium 235U, carbon 14C or tritium H3.