00493 IUPAC Gold Book - atom 10.1351/goldbook.A00493 A00493 current 1 Smallest particle still characterizing a chemical element. It consists of a nucleus of a positive charge (\(Z\) is the proton number and \(e\) the elementary charge) carrying almost all its mass (more than 99.9%) and \(Z\) electrons determining its size. chemical element https://goldbook.iupac.org/terms/view/C01022 elementary charge https://goldbook.iupac.org/terms/view/E02032 nucleus https://goldbook.iupac.org/terms/view/N04256 proton number https://goldbook.iupac.org/terms/view/A00499 Red Book, 3rd ed., p. 35 https://goldbook.iupac.org/terms/view/A00493/html https://goldbook.iupac.org/terms/view/A00493/json https://goldbook.iupac.org/terms/view/A00493/plain Citation: 'atom' 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.A00493 The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org . 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. 2026-06-28T05:01:10+00:00