In chemistry, free radicals are very reactive atoms or fragments of a molecule with one or more unpaired electrons.

These unpaired electrons are highly reactive, so free radicals are likely to take part in strong chemical reactions. In written chemical equations, free radicals are frequently denoted by a dot placed immediately to the right of the atomic symbol or molecular formula as follows:

H₂ + hν -> 2 H· (1)

Reactions involving free radicals are usually divided into three categories: intiation, propagation, and termination. Initiation reactions are those which result in a net increase in the number of free radicals. They may involve the formation of free radicals from stable species as in Reaction 1 above or they may involve reactions of free radicals with stable species to form more free radicals. Propagation reactions are those reactions involving free radicals in which the total number of free radicals remains the same. Finally, termination reactions are those reactions resulting in a net decrease in the number of free radicals. Typically, two free radicals combine to form a more stable species:

2H· -> H₂ (2)

Probably the most familiar free-radical reaction for most people is combustion. In order for combustion to occur the relatively strong O=O double bond must be broken to form oxygen free radicals. The flammability of a given material is strongly dependent on the concentration free radicals that must be obtained before initiation and propagation reactions dominate leading to combustion of the material. Once the combustible material has been consumed, termination reactions again dominate and the flame dies out.

In addition to combustion, many polymerization reactions involve free radicals. As a result many plastics, enamels, and other polymers are formed through free-radical reactions.

Relatively stable, persistent free radical compounds include [Fremys salt]? (Potassium nitrosodisulfonate, (KSO₃)₂NO·)and nitroxides?, (general formula R₂NO·).

A widely-used technique for studying free radicals, and other paramagnetic species, is [electron spin resonance]? spectroscopy (ESR). This is alternately referred to as "electron paramagnetic resonance" (EPR) spectroscopy.

It is conceptually related to nuclear magnetic resonance, though electrons resonate with higher-frequency fields at a given fixed magnetic field than do most nuclei.