Radical, in Chemistry, is a term applied to a group consisting of two or more elementary atoms which is capable of entering into a series of different compounds without itself undergoing change or decomposition. In this respect a radical resembles an atom of an elementary substance. Radicals are, in a sense, incapable of a separate existence, and must be in combination with other radicals or elements. Two similar radicals can, however, combine with each other, and in such cases the compound produced has been looked upon as the radical in the free state.
The radical methyl, , consisting of one atom of carbon and three atoms of hydrogen, is known in combination with chlorine, iodine, oxygen, and sulphur in the respective compounds, methyl chloride, , methyl iodide, , methyl oxide, , and methyl sulphide, . Two methyl groups occur in combination with each other in the substance ethan or dimethyl, or . A very large number of other compounds contain the radical methyl.
The sulphates, the nitrates, and the acetates contain the respective radicals , , and . These are frequently called the salt-radicals of the respective series of salts. In double decompositions the salt-radical occurring in combination with one metal is transferred, without decomposition, to another metal. For instance, the equation
represents the mutual exchange of salt-radicals by the metals magnesium and barium.
In chemical notation short symbols are frequently employed to represent complicated radicals. Thus, the complex salt-radical of the tartrates, , is shortly represented by the symbol . Tartaric acid is then represented by , potassium tartrate by , potassium hydrogen tartrate (cream of tartar) by , &c.