The existence of a chemical compound in two or more forms that are identical with respect to percentage composition but differ as to the positions of one or more atoms within the molecules, as well as in physical and chemical properties.
- geometric i. a form of i. displayed by unsaturated or ring compounds where free rotation about a bond (usually a carbon-carbon bond) is restricted; e.g., the i. of a cis- or trans- compound as in oleic acid and elaidic acid. Cf.:cis-, entgegen, trans-, zusammen.
- optic i. stereoisomerism involving the arrangement of substituents about an asymmetric atom or atoms (usually carbon) so that there is a difference in the behavior of the various isomers with regard to the extent of their rotation of the plane of polarized light. Cf.:stereoisomerism.
- structural i. i. involving the same atoms in different arrangements; e.g., the butyric acid s, leucine and isoleucine, glucose and fructose.

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isom·er·ism ī-'säm-ə-.riz-əm n
1) the relation of two or more chemical species that are isomers
2) the relation of two or more nuclides with the same mass numbers and atomic numbers but different energy states and rates of radioactive decay

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isom·e·rism (i-somґə-riz-əm) [iso- + Gr. meros part] the relationship that exists between two or more different chemical compounds that have the same molecular formula; it is divided into two broad classes: constitutional i. and stereoisomerism (q.v.). isomeric adj

Medical dictionary. 2011.

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