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

Question

Emilia Alvarez · Accepted Answer

#"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 #3s# and #3p# orbitals combined (the octet "rule").

However, since phosphorus can additionally access the #3d# orbitals, it gains more quantum states for the electron. The #3d# 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# #3d# orbitals, and #2xx5 = 10# electrons).

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

From this, we can say that phosphorus places its extra #2# electrons in the #3d# 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:

#"ClF"_5# (square pyramidal)
#"SF"_6# (octahedral)

Santiago Alfonso · Answer

undefined Examples of Lewis structures that violate the octet rule include molecules with an odd number of electrons, such as nitrogen dioxide (NO2), and molecules with expanded octets, such as sulfur hexafluoride (SF6) and phosphorus pentachloride (PCl5).