Free Radical

A free radical is any species capable of independent existence (hence the term free) that contains one or more unpaired electrons. An unpaired electron is one that occupies an atomic or molecular orbital by itself. In biological systems, free radicals are often derived from oxygen, nitrogen and sulphur molecules. Free radicals have the potential to damage cells in the body. They can chemically interact with cell components such as DNA, which in turn destabilises the intracellular molecular components that must seek and steal an electron from another molecule, therefore triggering a large chain of free radical reactions.

Free Radicals in Biology and Medicine. Fifth edition. Barry Halliwell and John M.C. Gutteridge. Oxford University Press 2015.