Exceptional Configurations of Chromium and Copper

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The above configurations show that the electronic configurations of chromium and copper are slightly different from that of other elements. The electronic configurations for these atoms are expected to be:

Cr: [Ar]3d⁴ 4s² and Cu: [Ar] 3d⁹ 4s²

However, the actual configurations are:

Cr: [Ar]3d⁵ 4s¹ and Cu: [Ar] 3d¹⁰ 4s¹

This is attributed to the fact that half-filled and completely filled electronic configurations (which include d⁵, d¹⁰, f⁷, f¹⁴) have lower energy. Thus, to acquire increased stability, one of the 4s-electrons goes into the nearby 3d-orbitals so that 3d-orbitals get half-filled in Cr and completely filled in Cu configurations. The extra stability of half-filled and completely-filled electronic configurations is due to their (i) symmetrical arrangement and (ii) large exchange energy.