Electron count determined by atomic number and ion charge

Steps:

• Identify atomic numbers: Mg = 12, N = 7, Ne = 10, S = 16.
• For neutral atoms, electrons equal atomic number (Ne has 10).
• For cations, subtract positive charge from atomic number (Mg²⁺ has 12 - 2 = 10).
• For anions, add negative charge magnitude to atomic number (N⁻ has 7 + 1 = 8; S²⁻ has 16 + 2 = 18).

Why D is correct:

• S²⁻ gains two electrons to achieve noble gas configuration, resulting in 18 electrons per the formula: electrons = atomic number + |charge| for anions.

Why the others are wrong:

• A: Mg²⁺ loses two electrons, leaving 10.
• B: N⁻ gains one electron, totaling 8.
• C: Ne is neutral with 10 electrons.

Final answer: D