Symmetry number

{
  "id": "MLC-T-CHM-011729",
  "term": "Symmetry number",
  "field": "Chemistry",
  "definition": "The number of indistinguishable orientations that a molecule can achieve through simple rotation of the entire molecule. This integer value, symbolized as σ or s, is used in statistical mechanics to correctly calculate rotational partition functions and entropies by avoiding overcounting identical configurations. For example, the symmetry number for water (H2O) is 2, for ammonia (NH3) is 3, and for methane (CH4) is 12.",
  "subject": "Physical and Biophysical Chemistry",
  "see_also": [
    "chemical equilibrium",
    "partition function"
  ],
  "references": [
    "Green Book, 2nd ed., p. 40 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)",
    "PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 189 (https://doi.org/10.1351/pac199668010149)"
  ],
  "url": "https://mlchart.com/terminology/chemistry/symmetry-number/"
}