Why there are exemptions in writing the electronic configurations of Chromium and Copper ?

Explanation

1) Elements which have half-filled or completely filled orbitals have greater stability.
2) So in chromium and copper the electrons in 4s and 3d redistributes their energies to attain stability by acquiring half-filled and completely filled d-orbitals.
3 ) Hence the actual electronic configuration of chromium and copper are as follows.