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Guys I hope you like my previous Post i.e. 
& today I am going to discuss another important topic Aufbau Principle. Without wasting your time let us start… 

(Building up principle or Aufbau rule)

It states that in the ground state of atom or ion electrons fill atomic orbital of the lowest available energy levels before occupying higher levels.

In the ground state of an atom,the electrons are added progressively to the various orbitals in increasing order of energies starting from the orbital of lowest energy.

The increasing order of energies of the various orbitals is :

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d ,7p,…

Fig 1. Order of increasing energies 

Aufbau Principle is basically used to find out electronic configuration of various elements. Electronic configuration of some of the elements are as :

Fig 2.Electronic configuration of some basic elements 

Few exception of Aufbau Principle :

Some of the exceptions are as follows

Fig 3. Increasing order of energy in atomic orbitals

There are different exception for the aufbau principle basically this principle is not so famous because due to some limitation which are as follows :

1. It does not tell us which electron are to be removed when an ion is formed for an atom.

2. (n-1)d subshells & ns subshells lie quite closer together yet the former is somewhat higher in energy.

These are the few limitations of Aufbau Principle.
Hope you understand it, still any doubt ask us in comment box.