Electronic Configuration of Cr & Cu : 

        Using the Aufbau or build-up principle (schematically represented as below) of electrons in atomic orbitals, one can easily write theelectronic configuration of atoms. But there are few exceptions (e.g., Cr, Cu). This is due to the fact that the energies of 3d & 4s levels in Sc - Zn are so close that frequent exchange of electrons occur between them and then the electronic arrangement is controlled by the phrase "half-filled & full-filled orbits are highly stable".