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) Based on Aufbau Principle, for n=4 , No. of sub-shells = 4s, 3d, 4p =three
(b) Total number of electrons which can be present = 2+10+6=18
Total number of electrons with `m_(s)` value of -1/2=9