Radioisotopes, or radionuclides, are unstable forms of elemental matter either man-made or found in nature. They all undergo a spontaneous process of radioactive decay through the emission of alpha and beta particles, gamma rays, and more. All of the elements on the periodic table with atomic numbers greater than 83 are radioisotopes. There are over 800 known radioisotopes that have been identified, with an additional 275 isotopes existing in general from the 81 stable elements on the periodic table.
Isotopes are forms of an element with varying numbers of neutrons in the nucleus of the atom. As radioisotopes decay, they slowly transform into other isotopes of the same element through gaining or losing neutrons, and eventually they become other elements entirely. This depends on their decay rate, which is known as a half-life. Uses of radioisotopes often depend on their half-life, which is the length of time for half of the mass of a radioactive material to decay into another material. Carbon, which is stable at 12C and 13C, is a radioisotope at 8C or 14C, with carbon-14 having the slowest decay rate at a half-life of 5,700 years. For this reason, and, due to the fact that its found in nature, 14C is used to do carbon dating of fossils and human artifacts from ancient societies.
In an unstable atom the proton/neutron balance is slightly different from its stable form, resulting in the binding energy of the nucleus being out of balance. As elements get heavier, more neutrons must exist in the nucleus to balance out proton-proton repulsion forces. For instance, uranium-238 is stable, because it has 92 protons and 146 neutrons in a nucleus. Nuclear radioisotopes like uranium-235 are unstable, however, with 92 protons and 143 neutrons, so they very slowly decay, with a half-life of 700 million years. Forcing uranium-235 to decay at a faster rate by neutron bombardment results in the creation of a very unstable nucleus that essentially blows itself apart and starts a chain reaction known as fission.
Medical radioisotopes such as iodine also lie outside what is known as the band of stability, but, in this case, they offer beneficial characteristics. Iodine-131 has four extra neutrons than its stable counterpart, and has a half-life of eight days. Since iodine can be safely ingested, it is used in medicine as a form of tracer or imaging agent. Iodine-125 is also used by being directly injected into tumors in a procedure known as brachytherapy, to slowly destroy tumor cells through radiation, with a half-life of 60 days. Radioisotopes in medical applications also include iridium-192 with a half-life of 72 days, and palladium-103 with a half-life of 17 days.