Read Why Phosphorus Forms Both PCl3 and PCl5
Phosphorus's richer halide and hydride chemistry compared to nitrogen comes down to one structural fact — phosphorus has accessible d-orbitals for expanding past an octet, nitrogen doesn't.
Phosphorus: Allotropes, Hydrides & Halides is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Why P forms PCl₅ but N cannot form NCl₅
Phosphorus (period 3) has empty 3d orbitals available to accommodate the extra bonding pairs in PCl₅ (5 electron domains, sp₃d); nitrogen (period 2) has no accessible d-orbitals and caps at 4 domains, so it forms only NCl₃, never NCl₅ — this same d-orbital availability underlies most 'why does the heavier group-15 element do X but nitrogen doesn't' questions.
White phosphorus is the most reactive, but not the most stable
White phosphorus (discrete tetrahedral P₄ molecules, strained ~60° bond angles) is the MOST REACTIVE allotrope precisely because of that angle strain, while black phosphorus (a layered, polymeric structure) is the most THERMODYNAMICALLY STABLE — reactivity and thermodynamic stability point in opposite directions here, so read each question carefully for which property it's actually asking about.
Phosphine formation from multiple starting points
PH₃ can be generated by hydrolysing calcium phosphide (Ca₃P₂ + H₂O), by heating H₃PO₃ (which disproportionates), or by reacting white phosphorus with concentrated NaOH under an inert atmosphere — recognising all three routes as valid sources of PH₃ is a recurring theme, while H₃PO₄ heated alone does NOT release phosphine (it dehydrates to polyphosphoric acids instead).
OlympIQ exam lens
- ✓Phosphorus can exceed an octet (accessible 3d orbitals, PCl₅); nitrogen cannot (no accessible 2d orbitals, only NCl₃).
- ✓White phosphorus is the most reactive allotrope (angle strain); black phosphorus is the most thermodynamically stable.
- ✓PH₃ forms from calcium phosphide hydrolysis, from heating H₃PO₃, or from P₄ + hot concentrated NaOH — but not from heating H₃PO₄.
- ✓Ca₃P₂, CaC₂ and Mg₃N₂ hydrolyse to give phosphine, acetylene and ammonia respectively — a classic three-way water-hydrolysis match.
Don't confuse 'most reactive' with 'most stable' for phosphorus allotropes — white phosphorus is the most reactive (due to ring strain) while black phosphorus is the most thermodynamically stable; a question can ask for either property.
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