A free radical is a molecule, molecular fragment, or ion having one or more unpaired electrons, usually short-lived (less than 1 ms) and highly reactive. Free radicals can be produced by photolysis or pyrolysis in which a bond is broken without forming ions. In formulae, a free radical is conventionally indicated by a dot (for example, ·Cl).

 

The most stable free radicals are NO, NO₂, and molecular oxygen, O₂ (a diradical). Organic free radicals have a complete range of stability, those of relatively long life, such as triphenylmethyl, which exist in equilibrium with their dimeric forms in solution; and those of relatively short life, such as methyl, which demand special techniques for their study.

 

The free radicals which have only a transient existence, such as ·CH₃, ·C₂H₅, and ·OH, which usually occur only as intermediates in chemical reactions, can typically be prepared and studied directly only at low pressures of the order of 1 mm, when they may be transported from the place of preparation in a rapid streaming inert gas without suffering too many collisions with other molecules. They may be frozen and stabilized in glasses and other matrices.

 

Free radicals are known to be formed by ionizing radiation and thus play a part in deleterious degradation effects that occur in irradiated tissue. They also act as initiators or intermediates in oxidation, combustion, photolysis, and polymerization.