isotope

One of two or more atoms that have the same atomic number (same number of protons), but which contain a different number of neutrons, thus differing in their relative atomic mass. They may be stable or radioactive (as a radioisotope), naturally occurring, or synthesized. For example, hydrogen has the isotopes ²H (deuterium) and ³H (tritium). The term was coined by English chemist Frederick Soddy, a pioneer researcher in atomic disintegration.

Elements at the lower end of the periodic table have atoms with roughly the same number of protons as neutrons. These elements are called stable isotopes. The stable isotopes of oxygen include ¹⁶O, ¹⁷O, and ¹⁸O; those of carbon include ¹²C and ¹⁴C. Elements with high atomic mass numbers have many more neutrons than protons and are therefore less stable. It is these isotopes that are more prone to radioactive decay. Examples are ²³⁸U (uranium-238) and ⁶⁰Co (cobalt-60).