# What are examples of lewis structures that violate the octet rule?

Jan 20, 2016

${\text{PCl}}_{5}$ is a pretty nice example.

With five single bonds, phosphorus has $10$ valence electrons.

Due to the Pauli Exclusion Principle (two electrons cannot have entirely matching quantum numbers), normally only $8$ electrons are allowed in the $3 s$ and $3 p$ orbitals combined (the octet "rule").

However, since phosphorus can additionally access the $3 d$ orbitals, it gains more quantum states for the electron. The $3 d$ orbital, if fully utilized, has the potential to allow an extra $10$ electrons if it was ever desired ($2$ per orbital, ${m}_{l} = - 2 , - 1 , 0 , + 1 , + 2$, thus $5$ $3 d$ orbitals, and $2 \times 5 = 10$ electrons).

This is possible because phosphorus's $3 p$ orbitals are close enough in energy to the $3 d$ that it can access them if it needs to (notice how $3 d$ and $3 p$ have the same $n$).

From this, we can say that phosphorus places its extra $2$ electrons in the $3 d$ orbitals when it needs to become hypervalent (expand its valency) to make all the bonds that it is prompted to make.

Some other examples are:

${\text{ClF}}_{5}$ (square pyramidal)
${\text{SF}}_{6}$ (octahedral)