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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)#.