CBSE Class 9 Answered

K, L, M, N are four different types of shells each having different capacity to hold maximum number of electrons.

K = 2(1)² = 2

L = 2(2)² = 8

M = 2(3)²= 18

N = 2(4)²= 32

Though M shell has capacity to hold 18 electrons but it cannot hold more than 8 electrons.

So when the number of electrons in the M shell exceeds 8 electrons, the filling of N shell takes place.

This is the reason why the electronic configuration of calcium is 2, 8, 8, 2 and not 2 8, 10.