1. Symmetrical distribution of electrons: It is well known that symmetry leads to stability. The completely filled or half filled subshells have symmetrical distribution of electrons in them and therefore are more stable. Electrons in the same subshell 3 d have equal energy but different spatial distribution.
Consequently, their, shielding of one another, is relatively small and the electrons are more strongly attracted by the nucleus.
2. Exchange Energy: Whenever two or more electrons with the same spin are present in the degenerate orbital of subshell then stablizing affect arises. "These electrons tends to exchange their position and the energy related due to this exchange is called exchange energy". The number of exchanges that can take, place is maximum when the subshell is either half filled or fully filled. As a result of exchange energy is maximum and so is stability.
Exchange energy is the basis of Hund's rule in such a way electron have parallel spin as far as possible. In other words, the extra stability of half filled and fully filled subshell is due to
(i) relatively small shielding
(ii) smaller Coulombic repulsion energy
(iii) larger exchange energy
The number of exchanges that can take place in d4 configuration are as follows:
Total number of exchanges = 3 + 2 + 1 = 6
The number of exchanges that can take place in d5 configuration áre as follows:
