Editing Octet rule (section)

== Hypervalent molecules ==
{{Main|Hypervalent molecule}}

Main-group elements in the third and later rows of the periodic table can form hypercoordinate or  [[hypervalent molecule]]s in which the central main-group atom is bonded to more than four other atoms, such as [[phosphorus pentafluoride]], PF<sub>5</sub>, and [[sulfur hexafluoride]], SF<sub>6</sub>. For example, in PF<sub>5</sub>, if it is supposed that there are five true [[covalent bond]]s in which five distinct electron pairs are shared, then the phosphorus would be surrounded by 10 valence electrons in violation of the octet rule. In the early days of quantum mechanics, [[Linus Pauling|Pauling]] proposed that third-row atoms can form five bonds by using one s, three p and one d orbitals, or six bonds by using one s, three p and two d orbitals.<ref>L. Pauling ''The Nature of the Chemical Bond'' (3rd ed., Oxford University Press 1960) p.63. In this source Pauling considers as examples PCl<sub>5</sub> and the [[hexafluorophosphate|PF<sub>6</sub><sup>−</sup>]] ion. {{ISBN|0-8014-0333-2}}</ref> To form five bonds, the one s, three p and one d orbitals combine to form five sp<sup>3</sup>d [[hybrid orbital]]s which each share an electron pair with a halogen atom, for a total of 10 shared electrons, two more than the octet rule predicts. Similarly to form six bonds, the six sp<sup>3</sup>d<sup>2</sup> hybrid orbitals form six bonds with 12 shared electrons.<ref>R.H. Petrucci, W.S. Harwood and F.G. Herring, General Chemistry (8th ed., Prentice-Hall 2002) p.408 and p.445 {{ISBN|0-13-014329-4}}</ref> In this model the availability of empty d orbitals is used to explain the fact that third-row atoms such as phosphorus and sulfur can form more than four covalent bonds, whereas second-row atoms such as nitrogen and oxygen are strictly limited by the octet rule.<ref>Douglas B.E., McDaniel D.H. and Alexander J.J. ''Concepts and Models of Inorganic Chemistry'' (2nd ed., John Wiley 1983) pp.45-47 {{ISBN|0-471-21984-3}}</ref>

[[image:penta phos.svg|thumb|500px | center |5 resonance structures of phosphorus pentafluoride]]
However other models describe the bonding using only s and p orbitals in agreement with the octet rule. A [[Valence bond theory|valence bond]] description of PF<sub>5</sub> uses [[Resonance (chemistry)|resonance]] between different PF<sub>4</sub><sup>+</sup> F<sup>−</sup> structures, so that each F is bonded by a covalent bond in four structures and an ionic bond in one structure. Each resonance structure has eight valence electrons on P.<ref>Housecroft C.E. and Sharpe A.G., ''Inorganic Chemistry'', 2nd ed. (Pearson Education Ltd. 2005), p.390-1</ref> A [[molecular orbital theory]] description considers the [[HOMO/LUMO|highest occupied molecular orbital]] to be a non-bonding orbital localized on the five fluorine atoms, in addition to four occupied bonding orbitals, so again there are only eight valence electrons on the phosphorus.{{citation needed|date=March 2015}} The validity of the octet rule for hypervalent molecules is further supported by [[Hypervalent molecule#Bonding in hypervalent molecules|ab initio molecular orbital calculations]], which show that the contribution of d functions to the bonding orbitals is small.<ref>Miessler D.L. and Tarr G.A., ''Inorganic Chemistry'', 2nd ed. (Prentice-Hall 1999), p.48</ref><ref>Magnusson, E., J.Am.Chem.Soc. (1990), v.112, p.7940-51 ''Hypercoordinate Molecules of Second-Row Elements: d Functions or d Orbitals?''</ref>

Nevertheless, for historical reasons, structures implying more than eight electrons around elements like P, S, Se, or I are still common in textbooks and research articles.  In spite of the unimportance of d shell expansion in chemical bonding, this practice allows structures to be shown without using a large number of formal charges or using partial bonds and is recommended by the IUPAC as a convenient formalism in preference to depictions that better reflect the bonding.  On the other hand, showing more than eight electrons around Be, B, C, N, O, or F (or more than two around H, He, or Li) is considered an error by most authorities.