Question

Explain why AsF5 does not follow the octet rule? How/why are there exceptions to the octet rule?

Confused as to why certain structures can have an exception to the octet rule, such as trigonal bipyramidal and octahedral.

Answer 1

empty d orbitals, that's why

Explanation:

Arsenic has a noble gas configuration of:
`[Ar] 4s^2 3d^10 4p^3`

Since arsenic has n=4, then it has angular momentum quantum numbers in this shell of 0, 1, 2, 3,

so you'd never write the electron configuration like this
`[Ar] 4s^2 3d^10 4p^3 4d^0 4f^0`, but this shows that since n=4, there is an empty 4d orbital. I explain this in class sometimes like "since your building has a 4th floor, then that means it has a room called s, and p, and d, and f....there is nobody in the d or f room, but they are still there...just empty"

Basically these empty d orbitals can get involved in the hybridization process `(sp^3d^1, sp^3d^2)` to allow these central atoms to have an expanded octet.