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Aufbau Principle

Arrangement of electrons in various orbital in an atom is called the electronic configuration of the atom. The Aufau principle (Aufbau is a German word meaning
“building-up”) is the procedure applied to build up the ground state (minimum energy) electronic configuration by adding stepwise protons to the nucleus and electrons to the orbital stating with the hydrogen atom. The assignment of electrons to orbital’s is done in order of their decreasing stability taking care not to violate the Pauli exclusion principle. Therefore, according to Aufbau Principle :” In the ground state of an atom the electrons occupy the lowest energy orbital available to them,” This means that the electrons enter the orbital in the order of increasing energies he lowest energy available orbital being filled up first.              
Since the energy of an orbital is determined by the quantum number n and l the sequence of filling the orbital proceeds according to the following rules:
(i)    Orbital are filled in order of increasing value of n+l. For instance, 4s (n+l ) = 4+0=4) is filled before 3d (n+1=3+2=5).
(ii)    For the orbital having the same value of n+l the orbital having glower value of n is filled up first. For example, 2p (n+l =2+1=3) is filled before 3s (n+l =3+0=3) because 2p has lower value of n.

The general order of increasing energies of the orbital is:
                                            1s 2s 2p 3s 3p 4s 4p 5s 5p 6s 4f 5d 6p 7s 5f 6d 7p

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