Isotope, atom of a chemical element which have the same number of protons in the nucleus, but a different numbers of neutrons, i.e., having the same atomic number but different mass. Isotopes of an element have identical chemical but varying physical properties. Most elements have several stable isotopes, being found in nature as mixtures. A few elements have natural radioactive isotopes (radioisotopes), which are unstable, and others of these can be made by exposing stable isotopes to radiation in a reactor.
See also: Element.