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 top of the periodic table have atoms with few protons and roughly the same number of neutrons. These elements have at least one, often several 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 (those heavier than iron) have many more protons and require large numbers of neutrons to shield the repulsion between their positive charges. These elements often come with a whole range of isotopes, most of which are prone to radioactive decay. Examples are ²³⁸U (uranium-238) and ⁶⁰Co (cobalt-60). Elements from number 84 (polonium) onwards have no stable isotopes, that is, all their nuclei will decay, although the decay may be very slow.