03523 IUPAC Gold Book - light-atom anomaly 10.1351/goldbook.L03523 L03523 current 1 A dynamical effect that arises for a process $$\ce{A + B-C -> A-B + C}$$ when the species $\ce{A}$ is light (e.g. a hydrogen atom) compared to $\ce{B}$ and $\ce{C}$. The vibrational excitation of the product $\ce{A–B}$ is low, since the light atom $\ce{A}$ approaches to within the bonding distance of $\ce{BC}$ before the $\ce{C}$ atom retreats. The energy of reaction is therefore released as repulsion between $\ce{A–B}$ and $\ce{C}$, with the result that there is translational excitation of the products. PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 171 (https://doi.org/10.1351/pac199668010149) https://goldbook.iupac.org/terms/view/L03523/html https://goldbook.iupac.org/terms/view/L03523/json https://goldbook.iupac.org/terms/view/L03523/plain Citation: 'light-atom anomaly' 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.L03523 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. 2026-06-30T05:11:55+00:00