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.
Radical (from Latin radicis, genitive of radix "root") can refer to many different things and concepts.




  • Radical consonant, which involves the root of the tongue
  • Radical (Chinese character), part of a Chinese character under which it is indexed in a dictionary (also called bùshǒu 部首)
  • One of the three consonants in a Semitic trilateral or quadrilateral root


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