Title: isomer
Long Title: IUPAC Gold Book - isomer
DOI: 10.1351/goldbook.I03289
Status: current

Definition
One of several species (or molecular entities) that have the same atomic composition (molecular formula) but different line formulae or different stereochemical formulae and hence different physical and/or chemical properties.

Related Terms
- line formulae: https://goldbook.iupac.org//terms/view/L03561
- molecular entities: https://goldbook.iupac.org//terms/view/M03986
- molecular formula: https://goldbook.iupac.org//terms/view/M03987
- stereochemical formulae: https://goldbook.iupac.org//terms/view/S05974

Sources
- PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1129 (https://doi.org/10.1351/pac199466051077)
- PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2210 (https://doi.org/10.1351/pac199668122193)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/I03289/html
- json: https://goldbook.iupac.org/terms/view/I03289/json
- xml: https://goldbook.iupac.org/terms/view/I03289/xml

Citation: Citation: 'isomer' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.I03289
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Disclaimer: The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using.
Accessed: 2026-05-17T21:45:45+00:00