JEE Main · 2025 · 07 Apr 2025 · Shift 2 · Atomic Structure

The stability of a half-filled subshell springs from several cooperating effects. With one electron in each degenerate orbital the charge cloud is symmetrical and the electrons are kept apart, so the coulombic repulsion between them is small. Every spin being parallel, the number of exchange pairs is maximum and the exchange energy, which is stabilising, is large. The mutual shielding of these singly occupied orbitals is also relatively small, so each electron feels the nucleus a little more strongly. The statement that does not apply is the one about electrons in non-degenerate orbitals, since the orbitals involved are in fact degenerate. The genuine reasons are the symmetrical distribution of electrons, the smaller coulombic repulsion, the larger exchange energy and the smaller mutual shielding.