*isomer*

a. [ See Anisomeric. ] (Bot.) Having the number of floral organs unequal, as four petals and six stamens. [ 1913 Webster ]

n. [ See Isomeric. ] (Chem.) A compound which is isomeric with another body or compound; a compound having the same chemical composition as another compound; a member of an isomeric series. [ 1913 Webster +PJC ]

☞ An isomer may be a structural isomer, in which some of the atoms are bonded to different atoms (as in CH3.CH2.NH2 and CH3.NH.CH3), or an optical isomer, in which the number and types of chemical bonds are identical, but the arrangement of the atoms in three-dimensional space is different; the latter are also called stereoisomers. [ PJC ]

n. [ Iso- + -mere. ] (Zool.) 1. A homologous or corresponding part or segment. [ Webster 1913 Suppl. ]

2. (Chem.) same as Isomer. [ archaic ] [ Webster 1913 Suppl. +PJC ]

a. [ Iso- + Gr. me`ros part: cf. F. isomérique. ] (Chem.) Having the same chemical composition and molecular weight; having the same number of atoms of each kind in the molecule; -- said of chemical compounds with known chemical composition. This property used to be called metameric to distinguish it from other forms of isomerism. [ PJC ]

☞ Formerly the term isomeric applied also to comopounds having the same percentage chemical composition, even if the number of atoms in a molecule differed. In the 1913 dictionary isomeric was defined as:
“Having the same percentage composition; -- said of two or more different substances which contain the same ingredients in the same proportions by weight, often used with with. Specif.: (a) Polymeric; i. e., having the same elements united in the same proportion by weight, but with different molecular weights; as, acetylene and benzine are isomeric (polymeric) with each other in this sense. See Polymeric. (b) Metameric; i. e., having the same elements united in the same proportions by weight, and with the same molecular weight, but with a different structure or arrangement of the ultimate parts; as, ethyl alcohol and methyl ether are isomeric (metameric) with each other in this sense. See Metameric.” [ 1913 Webster ]

n. (Chem.) An isomer. [ R. ] [ 1913 Webster ]

n. (Chem.) The state, quality, or relation, of two or more isomeric substances. [ 1913 Webster ]

Physical isomerism (Chem.), a former term for optical isomerism.
optical isomerism (Chem.), the condition or relation of certain (isomeric) substances, which, while chemically identical (in that they have the same composition, the same molecular weights, and the same ultimate constitution), are yet physically different, as in their action on polarized light, as dextro- and lævo-tartaric acids, or the sugars galactose and glucose. In such compounds there is usually at least one unsymmetrical carbon atom and typically more than one. See Unsymmetrical. [ 1913 Webster +PJC ]