More than 270 stable isotopes occur in nature. About 50 other natural isotopes, including those of uranium and radium, are radioactive. These isotopes, which give off particles or radiations, are called radioisotopes. These heavy atoms decay into various isotopes until they eventually become stable isotopes of lead. The rate at which radioactive isotopes decay is measured by the half-life, or the time required for half the atoms in a sample to decay.
Every isotope has a specific half-life.
Some isotopes in the radioactive series decay slowly. Others decay much faster. Some have half-lives of a small fraction of a second. A few scattered radioactive isotopes that do not belong to the series exist among elements lighter than bismuth. In the early 1940's, during World War II, scientists developed processes for separating large quantities of various isotopes.