Aufbau Principle: Electrons first enter the low energy orbital and then gradually enter the high energy orbital.
From the diagram above :-
1s<2s<2p<3s<3p<4s<3d<4p<5s – – – – –
Or 1s22s22p63s23p64s23d104p65s2s2 Thus entering the e-orbital.
(n + l) Rule: The orbital which is less than the value of (n + l) is the lower energy orbital, in which e will enter first. If the values of the two orbitals (n + l) are the same then the orbital whose value is less than n is the low power orbital. E.g. 2s, 2p, 3p, 4s, 3d The value of n + l 2s -> 2 + 0 = 2
2p -> 2 + 1 = 3
3p -> 2 + 1 = 4
4s -> 2 + 0 = 4
3d -> 2 + 2 = 5
2s <2p <3p <4s <3d in order of power.