page_title: Hypervalent molecule
section_title: Alternative definition
hierarchical_section_title: Hypervalent molecule / Alternative definition
caption_attribution_description: English: Calculation of the valence electron equivalent for phosphate and orthonitrate
context_page_description: A hypervalent molecule is a molecule that contains one or more main group elements apparently bearing more than eight electrons in their valence shells. Phosphorus pentachloride, sulfur hexafluoride, chlorine trifluoride, the chlorite ion, and the triiodide ion are examples of hypervalent molecules.
context_section_description: Durrant has proposed an alternative definition of hypervalency, based on the analysis of atomic charge maps obtained from atoms in molecules theory.  This approach defines a parameter called the valence electron equivalent, γ, as “the formal shared electron count at a given atom, obtained by any combination of valid ionic and covalent resonance forms that reproduces the observed charge distribution”.  For any particular atom X, if the value of γ(X) is greater than 8, that atom is hypervalent.  Using this alternative definition, many species such as PCl₅, SO₄²⁻, and XeF₄, that are hypervalent by Musher's definition, are reclassified as hypercoordinate but not hypervalent, due to strongly ionic bonding that draws electrons away from the central atom.  On the other hand, some compounds that are normally written with ionic bonds in order to conform to the octet rule, such as ozone O₃,  nitrous oxide NNO, and trimethylamine N-oxide (CH₃)₃NO, are found to be genuinely hypervalent.  Examples of γ calculations for phosphate PO₄³⁻ (γ(P) = 2.6, non-hypervalent) and orthonitrate NO₄³⁻ (γ(N) = 8.5, hypervalent) are shown below.
caption: Calculation of the valence electron equivalent for phosphate and orthonitrate
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