Explain the Aufbau principle with an example.

(N/A) The word 'aufbau' in German means 'building up'. The building up of orbitals refers to the process of filling orbitals with electrons.
Principle: 'In the ground state of atoms,the orbitals are filled in the order of their increasing energies.'
The order in which orbitals are filled is: $1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, \ldots$
Lower energy orbitals are filled completely before higher energy orbitals. The electron capacity of orbitals is as follows:

Orbital	$s, p, d, f$
Max. electron capacity	$2, 6, 10, 14$

Example:
$Be$ $(Z=4)$: $1s^{2} 2s^{2}$
$F$ $(Z=9)$: $1s^{2} 2s^{2} 2p^{5}$
$Br$ $(Z=35)$: $1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{10} 4p^{5}$