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# What is Aufbau Principle?

The Aufbau principle highlights the way in which the electrons fill in the atomic orbitals of an atom in its ground state. in other words, the electrons are filled in the orbitals in the increasing order of the orbital energy level.

According to the Aufbau principle the atomic orbitals that have the lowest energy levels are occupied firstly and additionally the orbitals with higher energy are occupied later.

The word ‘Aufbau’ in German means ‘construct’ or ‘build up’.

## Salient Features of the Aufbau Principle

• Therefore according to the Aufbau principle, the lowest orbitals are first occupied by the electrons, in other words, the electrons first occupy the lowest energy orbitals, the electrons enter the orbitals having higher energies only when orbitals with lower energies have been completely filled.
• The energy of the orbital and its order follows the (n+1) rule.
• here the sum of the principal and azimuthal quantum numbers determines the energy level of the orbital.
• Lower (n+l) values correspond to lower orbital energies.
• If two orbitals share equal (n+l) values, and
• the orbital in which the n value is low has
• lower energy associated with it.
• The order of the orbitals where the electrons are occupied show the filling of its electrons in the sequence:
• 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, and so on.

## Exceptions

The electron configuration of chromium is [Ar]3d54s1 and not [Ar]3d44s2 (as suggested by the Aufbau principle).

This exception attributes to several factors , for instance the increased stability provided by half-filled subshells and the relatively low energy gap between the 3d and the 4s subshells.

In conclusion, the electronic configurations of few atoms disobey the Aufbau principle (depending on the energy gap between the orbitals).

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