A radioactive isotope, also known as a radioisotope, radionuclide, or radioactive nuclide, is any of several species of the same chemical element with different masses whose nuclei are unstable and dissipate excess energy by spontaneously emitting radiation in the form of alpha, beta, and gamma rays. Every chemical element has one or more radioactive isotopes. For example, hydrogen, the lightest element, has three isotopes, which have mass numbers 1, 2, and 3. Only hydrogen-3 (tritium), however, is a radioactive isotope; the other two are stable. More than 1,800 radioactive isotopes of the various elements are known. Some of these are found in nature; the rest are produced artificially as the direct products of nuclear reactions or indirectly as the radioactive descendants of these products. Each “parent” radioactive isotope eventually decays into one or at most a few stable isotope “daughters” specific to that parent.

There are several sources of radioactive isotopes. Some radioactive isotopes are present as terrestrial radiation. Radioactive isotopes of radium, thorium, and uranium, for example, are found naturally in rocks and soil. Uranium and thorium also occur in trace amounts in water. Radon, generated by the radioactive decay of radium, is present in air. Organic materials typically contain small amounts of radioactive carbon and potassium. Cosmic radiation from the Sun and other stars is a source of background radiation on Earth. Other radioactive isotopes are produced by humans via nuclear reactions, which result in unstable combinations of neutrons and protons. One way of artificially inducing nuclear transmutation is by bombarding stable isotopes with alpha particles.

Radioactive isotopes have many useful applications. In particular, they are central to the fields of nuclear medicine and radiotherapy. In nuclear medicine, tracer radioisotopes may be taken orally or be injected or inhaled into the body. The radioisotope circulates through the body or is taken up only by certain tissues. Its distribution can be tracked according to the radiation it gives off. In radiotherapy, radioisotopes typically are employed to destroy diseased cells. Radiotherapy commonly is used to treat cancer and other conditions involving abnormal tissue growth, such as hyperthyroidism. Beams of subatomic particles, such as protons, neutrons, or alpha or beta particles, directed toward diseased tissues can disrupt the atomic or molecular structure of abnormal cells, causing them to die. Medical applications use artificial radioisotopes that have been produced from stable isotopes bombarded with neutrons.