The nucleus of an element normally has a fixed number of protons and neutrons. Some elements exist in different forms, however, with varying numbers of neutrons in their nuclei. A radioisotope is an element that releases radioactivity. Its nucleus is unstable. It can only become stable by releasing radiation and energy in the form of heat and particles. These are split off from the nucleus. Radioisotopes such as iodine and cobalt are widely used in medicine. They are particularly helpful in the study of organ function.
Radioisotopes are known to have a steady rate of decay. This means that we can use them to accurately date fossilized remains. Carbon-14 is used in this way.
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