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: 1s1
Helium: 1s2
Lithium: 1s2, 2s1
Beryllium: 1s2, 2s2
Boron: 1s2, 2s2, 2p1
Carbon: 1s2, 2s2, 2p2
Nitrogen: 1s2, 2s2, 2p3
Oxygen: 1s2, 2s2, 2p4
Fluorine: 1s2, 2s2, 2p5
Neon: 1s2, 2s2, 2p6
Sodium: 1s2, 2s2, 2p6, 3s1
Magnesium: 1s2, 2s2, 2p6, 3s2
Aluminum: 1s2, 2s2, 2p6, 3s2, 3p1
Silicon: 1s2, 2s2, 2p6, 3s2, 3p2
Phosphorus: 1s2, 2s2, 2p6, 3s2, 3p3
Sulphur: 1s2, 2s2, 2p6, 3s2, 3p4
Chlorine: 1s2, 2s2, 2p6, 3s2, 3p5
Argon: 1s2, 2s2, 2p6, 3s2, 3p6
Potassium: 1s2, 2s2, 2p6, 3s2, 3p6, 4s1
Calcium: 1s2, 2s2, 2p6, 3s2, 3p6, 4s2

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: 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d4
Actual electronic configuration: 1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d5

For copper:
Expected electronic configuration: 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d9
Actual electronic configuration: 1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d10

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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