Why is the Aufbau Principle Important?

Q.1. Explain Aufbau Principle.

Aufbau Principle: Aufbau is a German word means 'build up' or 'construction'. According to this principle every electron enters the lowest possible energy state available.
In other words orbitals are filled in order of increasing energy level of different orbitals is given below.
1s < 2s < 2p < 3s < 3p < 4s < 3d < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s < 5f < 6d < 7p..........
This order can also obtained by (n+l) rule where n and l  are the principle and azimuthal quantum numbersw respectively.

 (n+l) Rule: According to this rule a new electron can enter the orbital where (n+l) is minimum. In case (n+l) has the same value for two or more orbitals, the new electrons enters the orbital where n is minimum. e.g.

(i). Suppose we have to determine whether the new electron will enter in 3d or 4s orbitals

      for 3d orbital (n+l) = (3+2) = 5

      for 4s orbital (n+l) = (4+0) = 4

Here (n+l) is minimum for 4s orbital filled earlier or than 3d orbitals.

(ii). Now suppose we have to determine whether the new electron will enter in 3d or 4p orbital.

       for 3d orbital (n+l) = (3+2) = 5

       for 4p orbital (n+l) = (4+1) = 5 

In this case 3d orbital having lower value of energy level p (n=3) than 4p (n=4) is filled first.

Exceptions: 

1. One electron is added in 5d orbital before any electron is added in 4f orbital. After it, 14 electron added in 4f orbital. Then the remaining 9 electron are added in 5d orbital. (Atomic Number = 57).
2. One or two electrons are added in 6d orbital before any electron is added in 5f orbital. After it, 14 electrons are added in 5f orbital. The remaining 9 electrons are added in 6d orbital.