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Answer 1 · 2016-01-06T21:30:56

$d$ orbitals are more stable when they are half filled or fully filled.

The copper element has an atomic number of $Z=29$ which means that according to the Aufbau principle, the electron configuration should be:

$""_29Cu:[Ar] 4s^(2)3d^(9)$

However, it was found that copper's electron configuration as:

$""_29Cu:[Ar] 4s^(1)3d^(10)$

The explanation to this is that $d$ orbitals are more stable when they are half filled or fully filled.

There is a similar example which is Chromium $Cr$ where the electron configuration is:

$""_24Cr:[Ar] 4s^(1)3d^(5)$ instead of $""_24Cr:[Ar] 4s^(2)3d^(4)$ .