Term: symmetry number, \(s\)
https://doi.org/10.1351/goldbook.S06214

Definition:
The symmetry number of a molecule is obtained by imagining all identical atoms to be labelled, and then counting the number of different but equivalent arrangements that can be obtained by rotating (but not reflecting) the molecule. In the statistical-mechanical treatment of chemical equilibrium, the partition function for each molecular species must be divided by its symmetry number.

Related Terms:
1) chemical equilibrium (http://doi.org/10.1351/goldbook.C01023).
2) partition function (http://doi.org/10.1351/goldbook.P04439).

Source: Green Book, 2nd ed., p. 40 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)

Citation: 'symmetry number, \(s\)' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.S06214

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