Final answer:
Nitrogen does not form NF5 because it lacks d orbitals, allowing only for three bonds and a lone pair, while elements like phosphorus can expand their octet and form five bonds due to having d orbitals.
Step-by-step explanation:
Although PF5 and AsF5 are stable, nitrogen does not form NF5 molecules. The valence shell electron configuration of nitrogen has only four orbitals (one 2s and three 2p), which allows for the formation of up to three bonds with fluorine (as in NF3) plus one lone pair. In contrast, phosphorus and arsenic have vacant d orbitals in their valence shells, which can hybridize to form five bonds, as seen in PF5 and AsF5. Nitrogen lacks these d orbitals and thus cannot expand its octet to accommodate five bonds as in NF5.
Nitrogen does not form NF5 molecules because it does not have d orbitals to bond with the additional two fluorine atoms. In contrast, phosphorus can form NF5 molecules because it has d orbitals that can be used to bond with the fluorine atoms. Therefore, while PF5 and AsF5 are stable, nitrogen cannot form NF5 molecules due to its lack of d orbitals.