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Answer 1 · 2016-01-06T21:30:56

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

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

$_{29} C u : [A r] 4 s^{2} 3 d^{9}$ $""_29Cu:[Ar] 4s^(2)3d^(9)$

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

$_{29} C u : [A r] 4 s^{1} 3 d^{10}$ $""_29Cu:[Ar] 4s^(1)3d^(10)$

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

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

$_{24} C r : [A r] 4 s^{1} 3 d^{5}$ $""_24Cr:[Ar] 4s^(1)3d^(5)$ instead of $_{24} C r : [A r] 4 s^{2} 3 d^{4}$ $""_24Cr:[Ar] 4s^(2)3d^(4)$ .