# Aufbau Principle

According to this principle: “Filling of electrons always occurs from lower energy level to higher energy level. The orbital with lower energy level are filled up first. Only after then the orbitals with higher energy are filled.

So the electronic configuration of first 20 elements becomes:

Hydrogen: 1s^{1}

Helium: 1s^{2}

Lithium: 1s^{2}, 2s^{1}

Beryllium: 1s^{2}, 2s^{2}

Boron: 1s^{2}, 2s^{2}, 2p^{1}

Carbon: 1s^{2}, 2s^{2}, 2p^{2}

Nitrogen: 1s^{2}, 2s^{2}, 2p^{3}

Oxygen: 1s^{2}, 2s^{2}, 2p^{4}

Fluorine: 1s^{2}, 2s^{2}, 2p^{5}

Neon: 1s^{2}, 2s^{2}, 2p^{6}

Sodium: 1s^{2}, 2s^{2}, 2p^{6}, 3s^{1}

Magnesium: 1s^{2}, 2s^{2}, 2p^{6}, 3s^{2}

Aluminum: 1s^{2}, 2s^{2}, 2p^{6}, 3s^{2}, 3p^{1}

Silicon: 1s^{2}, 2s^{2}, 2p^{6}, 3s^{2}, 3p^{2}

Phosphorus: 1s^{2}, 2s^{2}, 2p^{6}, 3s^{2}, 3p^{3}

Sulphur: 1s^{2}, 2s^{2}, 2p^{6}, 3s^{2}, 3p^{4}

Chlorine: 1s^{2}, 2s^{2}, 2p^{6}, 3s^{2}, 3p^{5}

Argon: 1s^{2}, 2s^{2}, 2p^{6}, 3s^{2}, 3p^{6}

Potassium: 1s^{2}, 2s^{2}, 2p^{6}, 3s^{2}, 3p^{6}, 4s^{1}

Calcium: 1s^{2}, 2s^{2}, 2p^{6}, 3s^{2}, 3p^{6}, 4s^{2}

There are some exception of Aufbau’s principle. This principle does not hold true for chromium, copper, molybdenum, silver, aluminum.

*For chromium:*

Expected electronic configuration: 1s^{2}, 2s^{2}, 2p^{6}, 3s^{2}, 3p^{6}, 4s^{2}, 3d^{4}

Actual electronic configuration: 1s^{2}, 2s^{2}, 2p^{6}, 3s^{2}, 3p^{6}, 4s^{1}, 3d^{5}

*For copper:*

Expected electronic configuration: 1s^{2}, 2s^{2}, 2p^{6}, 3s^{2}, 3p^{6}, 4s^{2}, 3d^{9}

Actual electronic configuration: 1s^{2}, 2s^{2}, 2p^{6}, 3s^{2}, 3p^{6}, 4s^{1}, 3d^{10}

**Note:**Aufbau is a German word which means “building up”. So here, building up means building up of elements by gradual addition of electrons.

**Note:** Half filled and full filled orbitals are more stable because:

- Half filled orbitals have symmetry among the orbitals.
- There is a exchange of energy between the electrons.

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