isomer

/uy"seuh meuhr/, n.

1. Chem. a compound displaying isomerism with one or more other compounds.

2. Also called nuclear isomer. Physics. a nuclide that exhibits isomerism with one or more other nuclides.

[1865-70; back formation from ISOMERIC]

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One of two or more substances with identical molecular formulas but different configurations, differing only in the arrangement of their component atoms.

It usually refers to stereoisomers (rather than constitutional isomers or tautomers; see isomerism, tautomerism), of which there are two types. Optical isomers, or enantiomers (see optical activity), occur in mirror-image pairs. Geometric isomers are often the result of rigidity in the molecular structure; in organic compounds, this is usually due to a double bond (see bonding) or a ring structure. In the case of a double bond between two carbon atoms, if each has two other groups bonded to it and all are rigidly in the same plane, the corresponding groups can be on the same side (cis) of the C=C bond or across the C=C bond (trans) from each other. An analogous distinction can be made for ring structures that are all in a plane, between isomers whose substituent groups are on the same side and isomers whose substituent groups are on both sides of the plane. Diastereomers that are not enantiomers also fall into this category. Most cis-trans isomers are organic compounds.

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▪ nuclear physics

      in nuclear physics, any of two or more nuclides (species of atomic nuclei) that consist of the same number of protons and the same number of neutrons but differ in energy and manner of radioactive decay, and that exist for a measurable interval of time. The half-life of the more energetic isomer may be as short as about 10^-11 second but, in some extreme cases, as long as several years. Two nuclear isomers of cobalt-58, for example, are known: the lower energy isomer, ⁵⁸Co, of 71-day half-life (which decays by electron capture and positron emission); and the high-energy isomer, ^58mCo (m for metastable), of 9-hour half-life (which undergoes gamma decay, forming ⁵⁸Co).

      Nuclear isomers are formed as a direct result of reactions such as bombardment of nuclei by subatomic particles or as intermediate decay products of radioactive nuclei. Extremely unstable nuclei that decay as soon as they are formed in nuclear reactions and intermediate decay products the half-lives of which are less than about 10^-11 second are not generally classified as nuclear isomers.

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