How many electrons are there in the M shell of copper?

Think it through first, then pick an option.

Answer and explanation

Correct answer: d 18

Table 3.07: Cu is 2, 8, 18, 1; M holds 18.

Why the other options are wrong

a 9: 9 would follow the simple rule (3d⁹); in fact Cu is 2, 8, 18, 1, so M holds 18.
b 10: 10 is only the 3d electrons; the M shell has 3s² + 3p⁶ + 3d¹⁰ = 18.
c 17: 17 would be the count for 3d⁹ (2 + 6 + 9); copper's M shell has 18.