Question

Question #39125

Answer 1

Because M shell has the capacity for `8` electrons. Secondly, `18`
is used for all the sub shells in the M shell.

Explanation:

M shell has the capacity to store `8` electrons.
We have divided `18` in three sub shells of the shell M ,e.g.,
`3s = 2`
`3p = 6`
`3d = 10`
Now add them,
`2+6+10` = `18`
Electronic configuration of `Ca` =
`1s^2, 2s^2,2p^6, 3s^2,3p^6, 4s^2`, where;
`K = 2`
`L = 8`
`M = 8`
`N = 2`

You can take `18` as a whole but, if you'll take the M shell it contains 8 electrons, if we'll also count its sub shell electro configuration then it will be greater than 8.