n. In chem., identity or close similarity of composition and molecular weight, with difference of physical or of both chemical and physical properties.n. In the widest sense this term is applied to any two or more chemical compounds having the same percentage composition. Various degrees and kinds of isomerism are distinguished, the primary distinction being between chemical and physical isomerism. Chemical isomerism may be subdivided as follows: Polymerism, which is applied to compounds with the same percentage composition, but with molecular weights differing by some constant value: example, ethylene. C2H4, and butylene, C4H8, molecular weights 28 and 56 respectively. Polymerism may be either accidental or generic, according to whether the compounds belong to the same or different classes. Metamerism is applied to compounds with the same percentage composition and molecular weight, but containing homologous radicals in the molecule: examples are propylamine, C3H7NH2, ethylmethylamine, C2H5NHCH3, and trimethylamine, N(CH3)3. Chain or nucleus isomerism characterizes compounds which have the same number of similar atoms in the molecule, but in which the carbon atoms forming the nucleus are differently grouped: as, normal butane, CH3CH2CH2CH3, and isobutane, (CH3)3CH. Position or place isomerism exists in compounds which have similar nuclei but differ in the relative position of certain substituting atoms or radicals in this nucleus: as, primary or α-propyl chlorid, CH3CH2CH2Cl, and secondary or β-propyl chlorid, CH3CHClCH3. Structural isomerism applies to compounds which have the same percentage composition and molecular weight but differ in the arrangement of atoms in their molecules. Physical isomerism characterizes compounds which consist of varying aggregations of chemically similar molecules. It was first used of substances which are now termed geometrical, stereo-isomeric, or optically isomeric. Alloisomerism is a term occasionally applied to geometrically isomeric compounds one of which can pass into the other by the action of heat Dynamical isomerism is applied to compounds which have perfectly similar linkage and spatial arrangement of atoms, but which differ in energy content, that is. in atomic motion. (Tanatar, Ann. Chem. (Liebig), 273, 55.) Geometrical isomerism or stereo-isomerism marks compounds which have the same constitution, but which differ in the spatial arrangement of certain constituent radicals or atoms. Optical isomerism is applied to stereo-isomeric substances which differ in their action on a ray of polarized light: they either deflect it to the right (dextrorotatory), to the left (levorotatory), or are optically inactive (racemic or mesoform).
