a. How many sub-shell are associated with `n = 4`?
b. How many electron will be present in the sub-shell having `m_(s)` value of `-1//2` for `n = 4`?

Explanation

(a) For n =4 , No. of sub-shells `=(l=0, l=1, l=2, l=3)=4`.
(b) Total number of orbitals which can be present `=n^(2)=n^(2)=16`.
Each orbital can have an electron with `m_(s)=-1//2`
`:.` Total no. of electrons with `m_(s)=-1//2 ` is 16 .