Many of the radicals are not capable of in dependent existence, and hence cannot be iso lated. Others may be obtained in the free state, or in the form of a compound in which two similar radical groups are united to each other. When methyl iodide is with me tallic zinc the iodine is taken up by the zinc, and the methyl radical, CHs, is liberated. It is not obtained in the forth of methyl, however, for at the moment of its liberation two of the radicals combine to form a molecule of ethane, C,H.. Ethane, therefore, may be regarded as having the formula CH, CH., or as methyl hydride,° having the formula C,14,.H. It is convenient to regard it from one of these points of view for some purposes, and from the other for other purposes. A radical of the series tabulated herewith can exist in the free state, save for a slight change in its internal linkage, Provided its valency is an even number, and the free bonds ' belong to adjacent carbon atoms. Ethylene, for example, has the formula C,11,. When it exists as a radical, in combination with other atoms, it is divalent, and has the structure CH, the two carbon atoms being connected by one internal bond, while each carbon atom also has one bond which is satisfied by the ex ternal atoms with which tire radical is com bined. When the ethylene exists as a free
hydrocarbon, the two free bonds shown in the foregoing rational formula satisfy each other, and the constitution of free ethylene, therefore, CH, is II .
CH, The idea of a compound radical is due to Lavoisier ; and the conception, although its val idity was earnestly disputed for some years, has been of great importance in the development of chemistry. The definition of a radical, as given by Liebig in 1838 in his discussion of cyanogen, holds good at the present day. He said: eWe call cyanogen a radical because (1) it is the in variable constituent in a series of compounds; (2) because it can be replaced in these com pounds by other simple bodies; (3) because in its compounds with a simple body the latter can be replaced by equivalents of other simple bodies" At least two of these three character istics must be fulfilled by a group of atoms, be fore that group can be' regarded as a 'radical? Consult Hjelt, (General Organic Chemistry); Meyer, Theories of Chemistry.'