1) Aufbau principle is a scheme used to reproduce the electron configurations of the ground states of atoms by successively filling subshells with electrons in a specific order, filling subshells in the following order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f.
First exceptions to the Aufbau principle is for example chromium atom (Cr).
Chromium has atomic number 24, it means it has 24 protons and 24 electrons.
The Aufbau principle predicts the configuration [Ar] 3d⁴ 4s².
But the correct electron configuration of chromim atom is:
₂₄Cr 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹ or
[Ar] 3d⁵ 4s¹.
In chromium, half full d-sub level (3d) is more stable than a partially filled d sub-level (3d⁴), that is why an electron from the 4s orbital goes to a 3d orbital.
2) Second example is gallium atom (Ga).
Gallium has atomic number 31, it has 31 protons and 31 electrons.
The Aufbau principle predicts the configuration:
₃₁Ga 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p¹.
The correct electron configuration for gallium atom is:
₃₁Ga 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p¹.
That is because 3d subshell has lower energy, when is filled with electrons, than 4s subshell.
Valence electrons are in 4s and 4p orbitals, because when gallium loses three electrons, it has electron configuration:
₃₁Ga³⁺ 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰.
