The completely filled and completely halffilled sub-shells are stable due to thefollowing reasons:
- Symmetrical distribution ofelectrons:
It is well known that symmetryleads to stability. The completely filled orhalf filled subshells have symmetricaldistribution of electrons in them and are therefore more stable. Electrons in the same subshell have equal energybut different spatial distribution. Consequently, their shielding of one another is relatively small and the electrons are more strongly attracted by the nucleus.
The stabilizingeffect arises whenever two or more electrons with the same spin are present in the degenerate orbitals of a subshell.These electrons tend to exchange their positions and the energy released due to this exchange is called exchange energy.The number of exchanges that can takeplace is maximum when the subshell is either half filled or completely filled . As a result the exchange energy ismaximum and so is the stability.
You may note that the exchange energy is at the basis of Hunds rule that electrons which enter orbitals of equal energy have parallel spins as far aspossible.
In other words, the extra stability of half-filled and completely filled subshell is due to: (i) relatively small shielding, (ii) smaller coulombic repulsion energy, and (iii) larger exchange energy.