IIIustrate the reaasons for the stability of completey filled and half -filled subshells .

Explanation

Chromium and Copper shows anamalous electronic configurations
`Cr-[Ar]4s^(1)3d^(5)`
`Cu-[Ar]4s^(1)3d^(10)`
Cr-gets half filled 3d-shell electronic configuration.
gets full filled 3d-shell electronic configuration.
Halt filled and fullfilled subshells are more stable than others.
Causes of Stability of Completely filled and Half filled Sub-shells
The completely filled and half filled sub-shells are stable due to the following reasons :
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 are therefore more stable. Electrons in the same súbshell (here 3d) 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: The stabilizing effect 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 take place is maximum when the subshell is either half filled or completely filled. As a result the exchange energy is maximum and so is the stability.
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.