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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Phosphorus is a third‑period element and has empty 3d orbitals that can be used to accommodate more than eight valence electrons, so it can expand its octet and form five P–Cl bonds in PCl₅. Nitrogen is a second‑period element; its valence shell (n=2) contains only s and p orbitals, so it cannot expand beyond an octet. Consequently NCl₅ cannot be stable because nitrogen cannot accommodate the ten electrons required for five N–Cl bonds.

Examiner tips

  • State the key difference in valence‑shell capacity (d‑orbitals vs none).
  • Mention the need for five bonds and the 10‑electron requirement.
  • Use the term ‘octet expansion’ to show understanding.

Common mistakes

  • Confusing the role of d orbitals with the presence of a 3d subshell.
  • Assuming nitrogen can use d orbitals because it is in the third period.

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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