(a) n=4,l=1 (p-subshell), so 4p (b) n=4,l=0 (s-subshell), so 4s (c) n=3,l=2 (d-subshell), so 3d (d) n=3,l=1 (p-subshell), so 3p Accroding to the Bohr (n+l) rule, Enery order of the subshell : 3p<4s<3d<4p Note: When two orbital have the same value of (n+l) then the orbital which have lower value of n will be filled first.