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An isomer (; from Greek ἰσομερής, isomerès; isos = "equal", méros = "part") is a molecule with the same chemical formula as another molecule, but with a different chemical structure. That is, isomers contain the same number of atoms of each element, but have different arrangements of their atoms.〔Petrucci R.H., Harwood R.S. and Herring F.G. "General Chemistry" (8th ed., Prentice-Hall 2002), p. 91〕〔The word "isomer" was coined by Swedish chemist Jöns Jacob Berzelius (1779–1848) in 1830. See: Jac. Berzelius (1830) “Om sammansättningen af vinsyra och drufsyra (John’s säure aus den Voghesen), om blyoxidens atomvigt, samt allmänna anmärkningar om sådana kroppar som hafva lika sammansättning, men skiljaktiga egenskaper” (On the composition of tartaric acid and racemic acid (John's acid of the Vosges), on the molecular weight of lead oxide, together with general observations on those bodies that have the same composition but different properties) ''Kongliga Svenska Vetenskaps Academiens Handling'' (Transactions of the Royal Swedish Science Academy), vol. 49, pages 49–80; see especially (page 70 ). Reprinted in German in: J.J. Berzelius (1831) “Über die Zusammensetzung der Weinsäure und Traubensäure (John's säure aus den Voghesen), über das Atomengewicht des Bleioxyds, nebst allgemeinen Bemerkungen über solche Körper, die gleiche Zusammensetzung, aber ungleiche Eigenschaften besitzen," ''Annalen der Physik und Chemie'', vol. 19, pages 305–335; see especially (page 326 ). Reprinted in French in: J.J. Berzelius (1831) “Composition de l’acide tartarique et de l’acide racémique (traubensäure); poids atomique de l’oxide de plomb, et remarques générals sur les corps qui ont la même composition, et possèdent des proprietés différentes,” ''Annales de Chimie et de Physique'', vol. 46, pages 113–147; see especially (page 136 ).〕 Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like positional isomers, ''cis-trans'' isomers and enantiomers, etc. (see chart below). There are two main forms of isomerism: structural isomerism and stereoisomerism (spatial isomerism). ==Structural isomers== (詳細はfunctional groups are joined together in different ways. Structural isomers have different IUPAC names and may or may not belong to the same functional group.〔Smith, Janice Gorzynski. ''General, Organic and Biological Chemistry.'' The McGraw-Hill Companies. 1st ed 2010. pg. 450〕 This group includes chain isomerism whereby hydrocarbon chains have variable amounts of branching; position isomerism, which deals with the position of a functional group on a chain; and functional group isomerism, in which one functional group is split up into different ones. For example, two position isomers would be 2-fluoropropane and 1-fluoropropane, illustrated on the left side of the diagram above. In skeletal isomers the main carbon chain is different between the two isomers. This type of isomerism is most identifiable in secondary and tertiary alcohol isomers. Tautomers are structural isomers of the same chemical substance that spontaneously interconvert with each other, even when pure. They have different chemical properties and, as a consequence, distinct reactions characteristic to each form are observed. If the interconversion reaction is fast enough, tautomers cannot be isolated from each other. An example is when they differ by the position of a proton, such as in keto/enol tautomerism, where the proton is alternately on the carbon or oxygen. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Isomer」の詳細全文を読む スポンサード リンク
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