Exceptions to the Octet Rule

When drawing Lewis structures, there are several exceptions to the octet rule. There are radical species, species that form less than three bonds, and expanded valence shells.

Radical Species Most compounds have an even number of valence electrons. There are some exceptions, like NO₂. The total number of valence electrons is 5 for nitrogen plus 12 for oxygen which equals 17 valence electrons. The Lewis structure for NO₂ is below.

There is an unpaired electron on the nitrogen atom. Any species with an unpaired electron is a free radical. Radicals are highly reactive and can damage body tissues and cells because they will take electrons wherever they can get them. This is why we take antioxidants, like vitamin C, that provide electrons, in the form of hydride ion, to the free radicals. Free radicals are also important in the combustion of fuel, and in the preparation of polymers.

Species that are electron poor. The Lewis structure for BF₃ is shown below.

The boron atom only has three valence electrons and can only form 3 bonds. It cannot achieve an octet. Boron will react with other compounds because there is an empty p orbital that can accept a pair of electrons.

Expanded Valence Shells

Elements below the second period can accomodate more than an octet of electrons. For example, phosphorous can form either PCl₃ or PCl₅. The phosphorous atom in PCl₅ has 10 electrons surrounding it. In SF₆, the sulfur atom can accommodate 12 electrons because it is in the third period. Recall, the second period elements can only accommodate up to 8 electrons where in the third period there are the empty d orbitals meaning an atom can accommodate more than an octet of electrons. The Lewis structures for both PCl₅ and SF₆ are shown below.