00693 IUPAC Gold Book - Bohr radius 10.1351/goldbook.B00693 B00693 current constant 1 Atomic fundamental physical constant used as atomic unit of length: \[a_{0}=\frac{4\ \pi \ \varepsilon_{0}\ \mathrm{\hslash }^{2}}{m_{\rm{e}}\ e^{2}}=5.291 \ 772\ 49\ (24)\times 10^{-11}\ \rm{m}\] where \(e\) is the elementary charge, \(\mathrm{\hslash}\) is the Planck constant divided by \(2\ \pi \) and \(m_{\rm{e}}\) is the electron rest mass. Planck constant https://goldbook.iupac.org//terms/view/P04685 atomic unit https://goldbook.iupac.org//terms/view/A00504 electron rest mass https://goldbook.iupac.org//terms/view/E02008 elementary charge https://goldbook.iupac.org//terms/view/E02032 CODATA Bull. 1986, 63, 1 (https://doi.org/10.6028/jres.092.010) https://goldbook.iupac.org/terms/view/B00693/html https://goldbook.iupac.org/terms/view/B00693/json https://goldbook.iupac.org/terms/view/B00693/plain Citation: 'Bohr radius' 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.B00693 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. 2026-04-20T14:12:17+00:00