Aufbau Principle | Structure of Atom | Class 11

Here we have a given a easy notes on Aufbau Principle From Chapter Structure Of Atom.

   1. Aufbau Principle

•  Aufbau is a German word which means building up.
•  This principle tells the sequence in which various orbitals are filled with electrons.

Statement : Aufbau principle states that "In the ground state of the atoms the orbitals are filled with electrons in order of the increasing energies."

(n + l) rules :

• Electrons enter the orbitals according to following (n + l) rules.

Rule 1 : Orbitals are filled in order of increasing value of (n + l).

e.g. consider 3d and 4s orbitals

For 3d, n = 3 and l = 2.

Hence (n + l) = 3 + 2 = 5

For 4s, n = 4 and l = 0.

Hence (n+ l) = 4 + 0 = 4

As value of (n + l) for 4s-orbital is less than that 3d orbital, 4s orbital is filled before 3d orbital.

 

Rule 2 : ii) If two orbitals have same value of (n + l), then the orbital with lower value of n will be filled first.

e.g. consider 3d and 4p orbitals

For 3d, n = 3 and l = 2.

Hence (n + l) = 3 + 2 = 5

For 4p, n = 4 and l = 1.

Hence (n + l) = 4 + 1 = 5

Therefore, 3d orbital is filled up before 4p orbital.

• The order of energy of different orbitals in an atom is given below.

1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s < 5f < 6d < 7p, (8s, 5g, 6f, 7d, 8p, and 9s)