Title: normal mode
Long Title: IUPAC Gold Book - normal mode
DOI: 10.1351/goldbook.08648
Status: current

Definition
One of the \(3N - 6\) (\(3N - 5\) for a linear molecule) modes of vibration that would add to give the total vibrational motion in a molecule if the interatomic potential were strictly quadratic in the displacements from equilibrium.


Note
Normal modes of vibration is an idealized concept that has proved extremely useful in analysing spectra in vibrational spectroscopy.

Related Terms
- modes of vibration: https://goldbook.iupac.org//terms/view/08637
- vibrational spectroscopy: https://goldbook.iupac.org//terms/view/08708

Source
- PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 757 (https://doi.org/10.1515/pac-2019-0203)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/08648/html
- json: https://goldbook.iupac.org/terms/view/08648/json
- xml: https://goldbook.iupac.org/terms/view/08648/xml

Citation: Citation: 'normal mode' 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.08648
License: 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.
Collection: If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org .
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: 2026-05-31T05:03:39+00:00