Q.In degenerate orbitals, why do the completely filled and half filled configurations show more stability than the partially filled configuration?

Fully filled and exactly half-filled degenerate orbitals are extra stable mainly because of symmetrical electron distribution and maximised exchange energy, neither of which is at its maximum in a partially filled configuration.

Two related factors explain this extra stability:

1. Symmetrical distribution of electrons: When a set of degenerate (equal-energy) orbitals, such as the five d-orbitals or three p-orbitals, is either completely empty, exactly half filled (one electron in each, all with parallel spin), or completely filled (two electrons, paired, in each), the electron cloud around the nucleus is distributed with perfect symmetry. This symmetry minimises electron-electron repulsion and gives the atom greater stability than an unsymmetrical, partially filled arrangement (e.g. 3 out of 5 d-orbitals occupied), where the charge distribution is lopsided. …