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Some transition metals don't follow the aufbau, because half-filled orbitals and completely filled orbitals are associated with the stability of the element.

For example, For Cu with Z=29

If we follow aufbau, its condensed electronic config should be  [Ar] 4s² 3d⁹ . But because half-filled orbitals and complete orbitals are more stable, an electron from the s-sublevel will transfer to the d-sublevel. Thus, [Ar] 4s¹ 3d¹⁰. Here, the s-sublevel is half-filled and the d-sublevel is completely filled.
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