Occupied electron shells in ion configurations

Steps:

• Calculate electrons: Al³⁺ (13-3=10), Be²⁺ (4-2=2), N⁺ (7-1=6), S²⁻ (16+2=18).
• Assign configurations: Al³⁺ [Ne] (1s²2s²2p⁶); Be²⁺ 1s²; N⁺ 1s²2s²2p²; S²⁻ 1s²2s²2p⁶3s²3p⁶.
• Count shells (n values with electrons): Al³⁺ (n=1,2); Be²⁺ (n=1); N⁺ (n=1,2); S²⁻ (n=1,2,3).
• Compare: S²⁻ has the most (3 shells).

Why D is correct:

• S²⁻ configuration occupies n=1,2,3 per Aufbau principle, giving three shells with electrons.

Why the others are wrong:

• A. Al³⁺ has [Ne] configuration, occupying only two shells (n=1,2).
• B. Be²⁺ has 1s², occupying one shell (n=1).
• C. N⁺ has 1s²2s²2p², occupying two shells (n=1,2).

Final answer: D