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.
Phosphine Formation and Group 15 Halide Chemistry 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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