free radical, in chemistry, a molecule or atom that contains an unpaired electron but is neither positively nor negatively charged. Free radicals are usually highly reactive and unstable. They are produced by homolytic cleavage of a covalent bond (see chemical bond); i.e., each of the atoms connected by the bond retains one of the two electrons making up the bond. The homolytic cleavage of a hydrogen molecule, H2, produces two hydrogen free radicals (hydrogen atoms). Similarly, two chlorine free radicals can be produced from a chlorine molecule. Homolytic cleavage of the carbon-bromine bond in methyl bromide, CH3Br, would produce a methyl free radical and a bromine free radical. The term free is often dropped in referring to free radicals; this could lead to confusion if the term radical were used synonymously with group in organic chemistry, e.g., by calling an alkyl group an alkyl radical when free radical was not intended.
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