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

Explanation

The completely filled and half filled orbit-als give stability to the the atoms due to following reasons .
Completely filled and half filled orbitals give spherical symmetry to the atoms.
Also they give more number of Coulombic exchange energies .
These can be explained by taking the electronic cofigurations of Chromium and Copper as examples.
The expected configurations for Cr nas Cu are :
`Cr(Z=24)1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)4s^(2)3d^(4)`
` Cu(Z=29)1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)4s^(2)3d^(9)`
The two sub -shells 4s and 3d differ slightly in their energies . Then the electron shifts from a sub -shell of lower energy (4s) to s sub-shell of higher energy (3d) , provided such a shift results in all orbitals of the same sub-shell of higher energy (here, 3d) getting either completely filled or half filled . So the electronic configuration of Cr and Cu are
`Cr(Z=24)1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)4s^(1)3d^(5)`
` Cu(Z=29)1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)4s^(1)3d^(10)`
These configurations are more stable .
The completely filled and half - filled subshells are more stable for the following reasons :
1) There is symmetrical distribution of electrons in the sub-shells . so the they are more stable.

Each orbital has 1 electron They are half -filled
The 3d orbitals are completely filled
2) when two or more electrons with the same spin are present in the degenerate orbitals , these electrons exchange their positions . Then some energy is released , called 'exchange energy' . The the energy of the atom decreases and it becomes more stable . The more the number of 'exchange pair' of electrons , the greater would be the decrease of energy and hence the above configurations of Cr and Cu , there are moe 'exchange pairs' than in their previous configurations.


Possible exchange for a `d^(5)` configuration