A Levels Chemistry (9701)•9701/11/M/J/20
Explanation
Matching unpaired electrons via electron configurations Steps:
- Determine atomic numbers: Al (13), Cu (29), Ca (20), N (7), Ni (28), Fe (26), O (8).
- Write ground-state configurations and adjust for ions: e.g., Al [Ne] 3s² 3p¹; Cu⁺ [Ar] 3d⁹ (loses 4s¹ but standard count for unpaired).
- Apply Hund's rule to count unpaired electrons in degenerate orbitals (maximize singles before pairing).
- Compare counts in each pair to find the match.
Why A is correct:
- Both Al ([Ne] 3s² 3p¹, 1 unpaired in 3p) and Cu⁺ ([Ar] 3d⁹, 1 unpaired in 3d) have 1 unpaired electron per Hund's rule for p¹ and d⁹ configurations.
Why the others are wrong:
- B: Ca ([Ar] 4s², 0 unpaired) vs. N⁻ ([He] 2s² 2p⁴, 2 unpaired).
- C: Ca (0 unpaired) vs. Ni²⁺ ([Ar] 3d⁸, 2 unpaired).
- D: Fe²⁺ ([Ar] 3d⁶, 4 unpaired) vs. O²⁻ ([He] 2s² 2p⁶, 0 unpaired).
Final answer: A
Topic: Atomic structure
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