Number of occupied electron shells in ion configurations

Steps:

• Identify atomic numbers: Al (13), Be (4), N (7), S (16).
• Determine electron count and configuration for each ion: Al³⁺ (10 e⁻, [Ne]); Be²⁺ (2 e⁻, 1s²); N⁺ (6 e⁻, 1s² 2s² 2p²); S²⁻ (18 e⁻, [Ar]).
• Identify shells (n values) with electrons: count principal levels from n=1 up to the highest with ≥1 electron.
• Compare: maximum is three shells for S²⁻.

Why D is correct:

• S²⁻ has the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ (18 electrons), occupying three shells (n=1, 2, 3) per the Aufbau principle.

Why the others are wrong:

• A. Al³⁺ has 1s² 2s² 2p⁶ (10 electrons), occupying only two shells (n=1, 2).
• B. Be²⁺ has 1s² (2 electrons), occupying one shell (n=1).
• C. N⁺ has 1s² 2s² 2p² (6 electrons), occupying two shells (n=1, 2).

Final answer: D