radical, in chemistry, group of atoms that are joined together in some particular spatial structure and that take part in most chemical reactions as a single unit. Important inorganic radicals include ammonium, NH₄; carbonate, CO₃ ; chlorate, ClO₃, and perchlorate, ClO₄ ; cyanide, CN; hydroxide, OH; nitrate, NO₃; phosphate, PO₄; silicate, SiO₃ (meta) or SiO₄ (ortho); and sulfate, SO₄. The use of these radicals simplifies the naming and description of inorganic compounds, since such usage does not consider the electronic charge on the group. (When ions are dealt with, electronic charge must be considered.) In organic chemistry, the term radical is sometimes used synonymously with group; e.g., the group CH₃ is sometimes called the methyl radical instead of the methyl group. This use is limited chiefly to alkyl groups and aryl groups; it is usually not applied to functional groups, such as carbonyl. Because the term radical easily could be taken to mean a free radical, the term group is preferred by some.

radical, in mathematics, symbol placed over a number or expression, called the radicand, to indicate a root of the radicand. When used without a sign or index number, it designates the positive square root of the radicand, i.e., 2. If both square roots are meant, the radical sign is preceded by ±. To indicate higher roots of the radicand, e.g., cube or fourth roots, an index number is used. The radical sign is generally taken to indicate the principal root of the radicand, although any radicand will have n different nth roots. The term radical is sometimes used loosely to refer to the entire expression consisting of radical sign and radicand.

hydroxyl radical: see hydroxide.

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.

Term used in chemistry with one predominant and two subsidiary, looser meanings. It most often refers to a free radical. It can also mean an ion or a functional group.

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