(a) For a given value of ‘n’, ‘l’ can have values from zero to (n – 1). i.e., 0,1,2,3
The four sub-shells are associated with n = 4, which are s, p, d and f.
(b) For n=4
The number of orbitals will be = n2
The number of orbitals = 16.
Two electrons will go to each orbital with spins -1/2 and +1/2
∴ Number of electrons having spin -1/2 will be 16.