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. `n=4, l=0, 1, 2, 3`, (`4` subshelles, viz., `s, p, d, and f)`
b. Number of orbitals in `4th` shell `=n^(2)=4^(2)=16`
Each orbital has one electron with `m_(s)=-1//2`.
Hence, there will be `16` electrons with `m_(s)=-1//2`.