(a) How many sub-shells are associated with n = 4 ?

(b) How many electrons will be present in the sub-shells having ms value of -1/2 for n = 4 ?

When n = 4

l = 0,1,2,3.

l = 0; 4s- subshell

l = 1; 4p - subshell

l = 2;4d- subshell

l = 3; 4f - subshell

There are 4-subshells.

No. of electrons present in the subshells having ${{m}_{s}}$ value of 1/2 for n = 4 are 16.