# Explain why phosphorus(V) chloride (PCl₅) can exist as a stable molecule but nitrogen(V) chloride (NCl₅) cannot.

> IB DP Chemistry Higher Level (2023 syllabus) — S2.2 The covalent model · Explain · 4 marks

## Mark scheme (4 marks)

1. Phosphorus has available (empty) d orbitals in its valence shell that can be used for bonding, allowing expansion of the octet.
2. Nitrogen has no d orbitals in its valence shell (second period, n = 2), so it cannot expand beyond an octet.
3. PCl₅ requires phosphorus to form five covalent bonds, meaning the central atom must accommodate 10 electrons in its valence shell (octet expansion).
4. Therefore NCl₅ cannot exist because nitrogen cannot accommodate the 10 electrons that would be required to form five N–Cl bonds.

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