Hypervalent molecule

A hypervalent molecule is a molecule where an atom seems to have too many electrons or chemical bonds. Normally, this means a main-group element that has more than the eight valence electrons predicted by the octet rule.[1] An example of a hypervalent molecule is sulfuric acid, where the sulfur atom is drawn with six chemical bonds instead of four.

According to modern theories of bonding, these molecules do not really violate the octet rule. In molecular orbital theory, the electrons do not belong to the central atom, but shared by the outer atoms in a non-bonding orbitals. In valence bond theory, hypervalent molecules are resonance structures where the inner atom has a positive formal charge and the extra electrons are shared by the outer electrons.

In the case of sulfuric acid, the sulfur atom does not have any S=O double bonds: instead, the sulfur atom and non-acidic oxygen atoms have partial charges.[2] IUPAC rules prefer drawing the double bonds rather than showing partial charges, with the understanding that it represents resonance and not a true double bond.[3]

Sources

  1. ↑ International Union of Pure and Applied Chemistry. "Hypervalency". Compendium of Chemical Terminology Internet edition.
  2. ↑ Stefan, Thorsten; Janoschek, Rudolf (2000). "How relevant are S=O and P=O Double Bonds for the Description of the Acid Molecules H2SO3, H2SO4, and H3PO4, respectively?". Journal of Molecular Modeling. 6 (2): 282–288. doi:10.1007/PL00010730.
  3. ↑ Brecher, Jonathan (2008). "Graphical representation standards for chemical structure diagrams (IUPAC Recommendations 2008)". Pure and Applied Chemistry. 80 (2): 277–410. doi:10.1351/pac200880020277.
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