vsepr & hybridisation

Question

Which statement best explains why PF5 is stable while NF5 is not?

RankGuru · Accepted Answer

1. **Valence Shells**: 
   - Nitrogen is in the 2nd period: Electron configuration $[He] 2s^2 2p^3$. It has **no $d$ orbitals** in its valence shell.
   - Phosphorus is in the 3rd period: Electron configuration $[Ne] 3s^2 3p^3$. It has vacant, low-energy **$3d$ orbitals**.
2. **Octet Expansion**: To form $5$ covalent bonds, an atom must accommodate $10$ electrons in its valence shell (expanded octet).
3. **Outcome**: Phosphorus can promote an electron to the $3d$ orbital and form $sp^3d$ bonds. Nitrogen is strictly limited by the 'Octet Rule' and cannot form 5 bonds.
4. **Sterics**: Additionally, Nitrogen is so small that having 5 large atoms around it would cause extreme steric repulsion.

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