WHEN more then one orbital of equal energies are available the electrons will first occupy these orbitals singly with parallel spins. The pairing of electrons will then take place.
Every orbital in a subshell is singly occupied with one electron before any orbital is doubly occupied and all electron is singly occupied have the same spin.
|Fig 1. Electronic configuration with spin of some atoms.|
Hund’s rule can be stated in two ways:
a) In an atom no electron parining take place in the p,d or for vitals respectively until each orbital of the given sub-shell constrains one electron.
b) The unpaired electron present in the various orbital’s of the same sub-shell should have a parallel spin.
Explanation for the exceptional configuration :
- Symmetrical distribution of electrons
example : In case of Cr an electron gets shifted from 4s filled orbitals to 3d empty orbital to make all the five 3d orbitals half filled.
- Exchange energy
The extra stability of d configuration can also be explained on the basis of the exchange energy