Half filled and completely filled orbitals have extra stability. Why?

Explanation

The half filled and completely filled orbitals are extra stable due to the following reasons:
(i) Symmetrical distribution of electrons The completely filled and halffilled subshells have symmetrical distribution of electrons. Electrons in the same subshell have equal energy but different spatial arrangement. Due to symmetrical distribution, they become extra stable.
(ii) Exchange energy The stabilizing effect arises whenever two or more electrons with same spin are present in the degenerate orbitals of a subshell. They exchange their positions. Energy released due to this exchange is called exchange energy. The number of exchanges are maximum when subshell is either half filled or completely filled. As a result exchange energy is maximum and so the stability is also maximum. For example, in 3d there are less exchanges than the `3d^(5).`