How many electrons are in the 'M' shell of the Fe³⁺ ion?

Correct answer: c 13

Fe³⁺ (23 electrons): ...3s² 3p⁶ 3d⁵, so M = 2 + 6 + 5 = 13.

Why the other options are wrong

• a 2: 2 is the N shell of the Fe atom; in Fe³⁺ the N shell is empty and M holds 13.
• b 8: 8 would mean every 3d electron left; Fe³⁺ loses only 4s² and one 3d.
• d 14: 14 is the M shell of Fe or Fe²⁺; Fe³⁺ loses one more 3d electron, so 13.