Definitions

from The American Heritage® Dictionary of the English Language, 5th Edition.

  • noun The phenomenon of the existence of isomers.
  • noun The complex of chemical and physical phenomena characteristic of or attributable to isomers.
  • noun The state or condition of being an isomer.

from The Century Dictionary.

  • noun 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).
  • noun In chem., identity or close similarity of composition and molecular weight, with difference of physical or of both chemical and physical properties.

from the GNU version of the Collaborative International Dictionary of English.

  • noun (Chem.) The state, quality, or relation, of two or more isomeric substances.
  • noun (Chem.) a former term for optical isomerism.
  • noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

  • noun chemistry the phenomenon of the existence of isomers - the existence of different substances with the same molecular formulae
  • noun chemistry the interconversion of isomers

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • noun the state of being an isomer; the complex of chemical and physical phenomena characteristic of isomers

Etymologies

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Examples

  • The word isomerism was coined by Berzelius to express this anomalous condition of things, which seemed to negative the most fundamental truths of chemistry.

    A History of Science: in Five Volumes. Volume IV: Modern Development of the Chemical and Biological Sciences 1904

  • This phenomenon is called isomerism, and the generally accepted explanation is that the atoms in isomeric molecules are differently arranged, or grouped, in space.

    Four-Dimensional Vistas Claude Fayette Bragdon 1906

  • Sometimes, Mrs. Janner, it is important to remember that life is an isomerism in which the isomers change into one another with great ease so that they ordinarily exist together in equilibrium.

    Trisomy Jeffrey S. Callico 2011

  • The practice of identifying a chemical compound only by its composing elements was the norm in chemistry, until the discovery of isomerism by Friedrich Woehler in 1827.

    Natural Kinds Bird, Alexander 2008

  • The phenomenon of isomerism is often regarded as a success for atomism.

    Atomism from the 17th to the 20th Century Chalmers, Alan 2005

  • It was there I got my introduction to optical isomerism and the tetrahedral carbon atom.

    William S. Knowles - Autobiography 2002

  • He gave to us the basic methods of analysis which made chemistry an exact science, and endowed us with the pregnant ideas of isomerism and catalysis.

    Ronald G.W. Norrish - Banquet Speech 1968

  • Even in his earlier investigations into certain metal ammonias he was able to show that numerous cases of isomerism in complex cobalt and platinum compounds could only be satisfactorily explained by steric approach.

    Nobel Prize in Chemistry 1913 - Presentation Speech 1966

  • This is obviously important in such cases where we have to deal with chemical compounds of identical composition but of different structure, i.e. where isomerism is present.

    Nobel Prize in Chemistry 1936 - Presentation Speech 1966

  • There is no need for me to go into the familiar story of how isomerism was discovered; I should like to single out just one point.

    Wilhelm Ostwald - Nobel Lecture 1966

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