Illustrate the stability of half filled and completely filled orbitals with a suitable example.

Explanation

(a) Electronic configuration of chromium (Z =24) is `1s^(2) 2s^(2) 2p^(6) 3s^(2) 3p^(6)` but not `1s^(2) 2s^(2) 2p^(6)3s^(2)3p^(6)` . It is due to fact that half filled d- orbitals with all paraliel spins has extra stability. (b) Electronic configuration of copper (Z = 29) is `1s^(2) 2s^(2) 2p^(6) 3s^(2) 3p^(6)` but not `1s^(2) 2s^(2) 2p^(6) 3s^(2) 3p^(6)`
It is due to fact that completely filled d-orbitals with all spins paired is a more stable configuration, than the other one `(3d^(9) 4s^(2))`. This extra stability of half and completely filled orbitals can be explained in terms of symmetry and exchange energy.