Hypervalent molecules were first formally defined by Jeremy I. Musher in 1969 as molecules having central atoms of group 15–18 in any valence other than the lowest (i.e. 3, 2, 1, 0 for Groups 15, 16, 17, 18 respectively, based on the octet rule).
Several specific classes of hypervalent molecules exist:
* Hypervalent iodine compounds are useful reagents in organic chemistry (e.g. Dess–Martin periodinane)
* Tetra-, penta- and hexavalent phosphorus, silicon, and sulfur compounds (e.g. PCl, PF, SF, sulfuranes and persulfuranes)
* Noble gas compounds (ex. xenon tetrafluoride, XeF)
* Halogen polyfluorides (ex. chlorine pentafluoride, ClF)