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In chemistry, isomers (/ˈsəmərz/; frae Greek ἰσομερής, isomerès; isos = "equal", méros = "part") are molecules wi the same molecular formula but different chemical structures. That is, isomers contain the same nummer o atoms o each element, but hae different arrangements o thair atoms in space.[1][2] Isomers dae nae necessarily share seemilar properties, unless thay an aa hae the same functional groups. Thare are mony different classes o isomers, lik poseetional isomers, cis-trans isomers an enantiomers, etc. (see chart below). Thare are twa main furms o isomerism: structural isomerism an stereoisomerism (spatial isomerism).

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  1. Petrucci R.H., Harwood R.S. and Herring F.G. "General Chemistry" (8th ed., Prentice-Hall 2002), p.91
  2. 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.