Why Cr and Cu have exceptional electronic configurations ?

Explanation

The electronic configurations of Cr and Cu are different from those as expected. The expected electronic configurations of these atoms are : `Cr: [Ar]3d^(4) 4s^(2) and Cu : [Ar]3d ^(9)4s ^(2)` However the actual configurations are: `Cr: [Ar]3d ^(5) 4s^(1) and Cu: [Ar]3d^(10 4s^(1))` This is due to the fact that half filled and completely filled clectronic configurations have lower energy and extra stability. So to acquire maximum stability, one of the 4s electrons goes into the nearby 3d-orbitals. So that 3d-orbitals become half filled in Cr and completely filled in Cu.