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.
Read more about this topic: Octet Rule
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“Skepticism is unbelief in cause and effect. A man does not see, that, as he eats, so he thinks: as he deals, so he is, and so he appears; he does not see that his son is the son of his thoughts and of his actions; that fortunes are not exceptions but fruits; that relation and connection are not somewhere and sometimes, but everywhere and always; no miscellany, no exemption, no anomaly,but method, and an even web; and what comes out, that was put in.”
—Ralph Waldo Emerson (18031882)
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