Electron count determined by atomic number and ion charge

Steps:

• Identify atomic numbers: Mg (12), N (7), Ne (10), S (16).
• Neutral atoms have electrons equal to atomic number: Ne has 10.
• Cations lose electrons: Mg²⁺ loses 2, so 10 electrons.
• Anions gain electrons: N⁻ gains 1 (8 electrons), S²⁻ gains 2 (18 electrons).

Why D is correct:

• S²⁻ is the sulfide ion; by ion formation rules, sulfur (atomic number 16) gains 2 electrons to achieve noble gas configuration, totaling 18 electrons.

Why the others are wrong:

• A: Mg²⁺ loses 2 electrons from neutral magnesium (12), leaving 10.
• B: N⁻ gains 1 electron from neutral nitrogen (7), totaling 8.
• C: Ne is neutral, so 10 electrons match its atomic number.

Final answer: D