Explain Aufbau principle with example

Explanation

The word aufbau in German means building up L the building up of orbitals means the filling up of orbitals with electrons
Principle : in the ground state of the atom the orbitlas are filled in order of their increasing energies.
Order of orbitals are filled : 1s , 2s ,2p 3s, 3p , 4s ,3d ,4p 5s , 4d , 5p , 6s , 4f , 5d , 6p , 7s
First lower energy orbitals completely filled than after other orbitals are filled .The capacity of orbitals for electron is as follows

Ex : Be (z=2) electron configuration `1s^(2) 2s^(2) P (z=9)` electron configuration `1s^(2) 2s^(2) 2p^(5)`
Br (z=35) arrangement of electron as under
`1s^(2) 2s^(2) 2p^(6) 3s^(2) 4s^(2) 3d^(10) 4p^(5)` etc