How many electrons are in the M shell of ₂₅Mn²⁺?

Correct answer: d 13

Mn²⁺ (23 electrons): ...3s² 3p⁶ 3d⁵, so M holds 2 + 6 + 5 = 13.

Why the other options are wrong

• a 0: Mn²⁺ loses its two outer 4s electrons; the M shell is not emptied, it keeps 13.
• b 5: 5 counts only 3d⁵; the M shell holds 3s² + 3p⁶ + 3d⁵ = 13.
• c 8: 8 would need 3d electrons removed too; 4s goes first, so M keeps 13.