free radicals A free radical is any chemical species which contains one or more unpaired electrons and which is capable of an independent existence. An unpaired electron is one that can alone occupy an atomic or molecular orbital, conventionally denoted by a superscript dot: X^•.

The biologically important free radicals are the oxygen species, superoxide O^•₂^-, the hydroxyl radical OH^•, and the reactive nitrogen species NO^•; each may play a significant physiological or pathophysiological role in the body.

See ageing; nitric oxide; oxygen.

free radicals Highly reactive molecules with an unpaired electron. See antioxidant nutrients.