<P> Main - group elements in the third and later rows of the periodic table can form hypercoordinate or hypervalent molecules in which the central main - group atom is bonded to more than four other atoms, such as phosphorus pentachloride, PCl, and sulfur hexafluoride, SF . For example, in PCl, if it is supposed that there are five true covalent bonds 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, 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 . To form five bonds, the one s, three p and one d orbitals combine to form five sp d hybrid orbitals 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 d hybrid orbitals form six bonds with 12 shared electrons . 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 . </P> <P> However other models describe the bonding using only s and p orbitals in agreement with the octet rule . A valence bond description of PF uses resonance between different PF F 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. A molecular orbital theory description considers the 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 . The validity of the octet rule for hypervalent molecules is further supported by ab initio molecular orbital calculations, which show that the contribution of d functions to the bonding orbitals is small . </P>

When bonding hydrogen can have at most how many electrons in its valence shell