One of two or more species of atom s of a chemical element having nuclei with the same number of proton s but different numbers of neutron s.
They have the same atomic number and hence nearly identical chemical behaviour but different atomic mass es. Most elements found in nature are mixtures of several isotopes; tin, for example, has 10 isotopes. In most cases, only stable isotopes of elements are found in nature. The radioactive forms break down spontaneously into different elements (see radioactivity ). Isotopes of all elements heavier than bismuth are radioactive; some occur naturally because they have long half-lives .