The article of chemistry:Radioactive isotope

        Radioactive isotope, also called radioisotope, radionuclide, orradioactive nuclide, any of several species of the same chemical elementwith different masses whose nuclei are unstable and dissipate excess energy by spontaneously emitting radiation in the form of alpha, beta, and gamma rays.
        A brief treatment of radioactive isotopes follows. For full treatment, see isotope: Radioactive isotopes. Every chemical element has one or more radioactive isotopes. For example,hydrogen, the lightest element, has three isotopes with mass numbers 1, 2, and 3. Only hydrogen-3 (tritium), however, is a radioactive isotope, the other two being stable. More than 1,000 radioactive isotopes of the various elements are known. Approximately 50 of these are found in nature; the rest are produced artificially as the direct products ofnuclear reactions or indirectly as the radioactive descendants of these products.
         Radioactive isotopes have many useful applications. In medicine, for example,cobalt-60 is extensively employed as aradiation source to arrest the development of cancer. Other radioactive isotopes are used as tracers for diagnostic purposes as well as in research on metabolic processes. When a radioactive isotope is added in small amounts to comparatively large quantities of the stable element, it behaves exactly the same as the ordinary isotope chemically; it can, however, be traced with a Geiger counter or other detection device. Iodine-131 has proved effective in treatinghyperthyroidism. Another medically important radioactive isotope is carbon-14, which is used in a breath test to detect the ulcer-causing bacteriaHeliobacter pylori.
         In industry, radioactive isotopes of various kinds are used for measuring the thickness of metal or plastic sheets; their precise thickness is indicated by the strength of the radiations that penetrate the material being inspected. They also may be employed in place of large X-raymachines to examine manufactured metal parts for structural defects. Other significant applications include the use of radioactive isotopes as compact sources of electrical power—e.g., plutonium-238 inspacecraft. In such cases, the heatproduced in the decay of the radioactive isotope is converted into electricity by means of thermoelectric junction circuits or related devices.
The table lists some naturally occurring radioactive isotopes.