The molecules PF5 and AsF5 exist, but the analogous molecule NF5 does not. Why not?
The correct answer and explanation is :
The molecule NF₅ does not exist, unlike PF₅ (phosphorus pentafluoride) and AsF₅ (arsenic pentafluoride), due to a combination of factors related to atomic size, bonding capabilities, and the lack of sufficient orbital overlap.
- Atomic Size and Orbital Overlap: Nitrogen is much smaller than phosphorus and arsenic. In PF₅ and AsF₅, the central atoms, phosphorus, and arsenic, have larger atomic radii, which allows for better overlap of their valence orbitals with those of fluorine. This overlap is crucial for the formation of stable covalent bonds with fluorine. On the other hand, nitrogen’s smaller size means the overlap with fluorine’s orbitals would not be as effective. The resulting bonds would be weaker, and the molecule would not be stable.
- Bonding Constraints: Nitrogen has only five valence electrons and typically forms three bonds in compounds like NH₃ (ammonia). If it were to form five bonds, as in NF₅, nitrogen would need to accommodate more than its available valence electrons, creating an unstable situation. While phosphorus and arsenic can expand their valence shells (using their d-orbitals) to accommodate more than four bonds, nitrogen cannot do so. Therefore, the bonding in NF₅ would not be feasible.
- Electron Configuration: The nitrogen atom has a small 2p orbital, which is not as effective at accommodating additional bonding electrons compared to the larger 3p, 4p, or 5p orbitals of phosphorus and arsenic. This limited orbital availability further prevents the formation of stable NF₅.
In summary, the non-existence of NF₅ is due to nitrogen’s inability to expand its valence shell, the weak overlap of nitrogen’s smaller orbitals with fluorine’s orbitals, and the resulting instability of such a molecule.