/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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      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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Universalium. 2010.

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Look at other dictionaries:

  • Isomer — I so*mer ([imac] s[ o]*m[ e]r), 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] Note …   The Collaborative International Dictionary of English

  • Isomer — Isomēr (grch.), aus gleichen Teilen bestehend. Isomere Körper, in der Chemie Körper von gleicher prozentischer Zusammensetzung, aber verschiedenen Eigenschaften; die Isomeriē kann sich nur auf äußerliche Eigenschaften beziehen (physik. Isomerie) …   Kleines Konversations-Lexikon

  • isomer — ISOMÉR, Ă adj., s.m. v. izomer. Trimis de LauraGellner, 13.09.2007. Sursa: DN …   Dicționar Român

  • isomer — 1866, back formation from isomeric (1838), patterned on Ger. isometrisch, coined by Berzelius (1830) from Gk. isomeres sharing equality, from iso (see ISO (Cf. iso )) + meros part, share (see MERIT (Cf. merit) (n.)) …   Etymology dictionary

  • isomer — ► NOUN 1) Chemistry each of two or more compounds with the same formula but a different arrangement of atoms and different properties. 2) Physics each of two or more atomic nuclei with the same atomic number and mass number but different energy… …   English terms dictionary

  • isomer — [ī′sə mər] n. [< Gr isomerēs, equally divided < isos, equal + meros, a part: see MERIT] 1. Chem. any of two or more chemical compounds having the same constituent elements in the same proportion by weight but differing in physical or… …   English World dictionary

  • Isomer — This article is about the chemical concept. For isomerism of atomic nuclei, see nuclear isomer. In chemistry, isomers (from Greek ἰσομερής, isomerès; isos = equal , méros = part ) are compounds with the same molecular formula but different… …   Wikipedia

  • Isomer — Isomere (Einzahl das Isomer, Genitiv: des Isomers, Genitiv Plural: der Isomere; von griechisch ἴσος ísos „gleich“ und μέρος méros „Teil“) sind chemische Verbindungen, die die gleiche Summenformel besitzen, sich aber in der Verknüpfung und der… …   Deutsch Wikipedia

  • isomer — noun /ˈaɪ.sə.mə,ˈaɪ.sə.mɚ/ a) Any of two or more compounds with the same molecular formula but with different structure. b) Any of two or more atomic nuclei with the same mass number and atomic number but with different radioactive properties.… …   Wiktionary

  • isomer — iso|mer 〈Adj.〉 Isomerie aufweisend [<grch. isos „gleich“ + meros „Teil“] * * * I|so|mer [↑ iso (2) u. ↑ mer], das; s, e, auch I|so|me|re, das; n, n: 1) in der Chemie Sammelbez. haupts. für org. Verb. u. Koordinationsverb., die zwar gleiche… …   Universal-Lexikon

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