AQA A-Level Chemistry AS Paper 1, June 2025: Question 16

✅ Correct Answer

C — nickel

A neutral nickel atom has the configuration [Ar] 4s² 3d⁸ . The five 3d orbitals contain 8 electrons: three pairs and two singly occupied (unpaired) orbitals.

🧠 Exam Technique

• Find atomic numbers (Z) immediately from your Periodic Table:
  Co (Z = 27), Mg (Z = 12), Ni (Z = 28), P (Z = 15).
• Focus only on incomplete sub-shells: Completely filled shells (like [Ar] or 4s²) have zero unpaired electrons.
• Draw "electrons in boxes": Sketch the 5 boxes for the d sub-shell or 3 boxes for the p sub-shell to avoid quick counting mistakes under timed pressure.

A • Cobalt (Z = 27):
Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁷ (or [Ar] 4s² 3d⁷ )
In the 3d sub-shell (5 degenerate orbitals): 2 orbitals are paired (4 e⁻), and 3 orbitals are singly occupied (3 e⁻).
→ 3 unpaired electrons (Incorrect)

B • Magnesium (Z = 12):
Configuration: 1s² 2s² 2p⁶ 3s² (or [Ne] 3s² )
All sub-shells are completely filled; the 3s orbital holds a paired set of electrons.
→ 0 unpaired electrons (Incorrect)

C • Nickel (Z = 28):
Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁸ (or [Ar] 4s² 3d⁸ )
Distributing 8 electrons across five 3d orbitals according to Hund's rule:
Orbital 1: [↑↓], Orbital 2: [↑↓], Orbital 3: [↑↓], Orbital 4: [↑], Orbital 5: [↑]
Three orbitals contain pairs; two orbitals contain single electrons.
→ 2 unpaired electrons (CORRECT)

D • Phosphorus (Z = 15):
Configuration: 1s² 2s² 2p⁶ 3s² 3p³ (or [Ne] 3s² 3p³ )
In the 3p sub-shell (3 degenerate orbitals), each orbital holds one electron with parallel spins: [↑] [↑] [↑].
→ 3 unpaired electrons (Incorrect)

💡 Key Knowledge

• Hund's Rule: Orbitals of equal energy (degenerate orbitals) are each occupied singly with parallel spins before any orbital is doubly occupied.
• d-Orbital Capacity: The d sub-shell has 5 orbitals holding up to 10 electrons:
  • d¹ → 1 unpaired • d² → 2 unpaired
  • d³ → 3 unpaired • d⁷ → 3 unpaired
  • d⁸ → 2 unpaired • d⁹ → 1 unpaired
• Note that dⁿ and d¹⁰⁻ⁿ always have the same number of unpaired electrons!

❌ Common Traps & Misconceptions

• Mixing up Cobalt and Nickel: Students frequently miscount d⁷ as having 2 unpaired electrons by dividing 7 by 2 or confusing holes with electrons. Remember: 5 single electrons fill first, leaving only 2 to pair up → 3 remain unpaired.
• Thinking Phosphorus has 1 unpaired electron: Students sometimes mistakenly pair up two electrons in the 3p sub-shell ([↑↓] [↑] [ ]) instead of applying Hund's rule ([↑] [↑] [↑]).
• Confusing Atoms with Ions: Always verify whether the question asks for a neutral atom or a transition metal ion (e.g. Ni²⁺ vs Ni). Here, it asks specifically for neutral atoms.