Octet Rule - Exceptions

Exceptions

• The duet rule of the first shell - the noble gas helium has two electrons in its outer shell, which is very stable. (Since there is no 1p subshell, 1s is followed immediately by 2s, and thus shell 1 can only have at most 2 valence electrons). Hydrogen only needs one additional electron to attain this stable configuration, while lithium needs to lose one.
• Trivalent boron compounds such as BF₃ have only 6 electrons in the valence shell, as do some reactive species such as carbenes. These molecules often react so as to complete their octet: trivalent boron compounds are well known as Lewis acids which form a fourth bond with a Lewis base, and carbenes are even more reactive. Beryllium and aluminium can also have incomplete octets.
• Free radicals (e.g. nitric oxide) contain one or more atoms which have an odd number of electrons.
• Hypervalent molecules in which main group elements exhibit more than four bonds, for example phosphorus pentachloride, PCl₅, and sulfur hexafluoride, SF₆. The bonding in such molecules has been controversial. One model considers that the P and S atoms (in PCl₅ and SF₆ respectively) form five and six true covalent bonds with the participation of d orbitals, in violation of the octet rule. However another model describes such molecules with three-center four-electron bonds and conforms to the octet rule. This second model is supported by ab initio molecular orbital calculations which show that the contribution of d functions to the bonding orbitals is small.
• For transition metals, the 18-Electron rule replaces the octet rule, due to the importance of d orbitals in these atoms.