Question

Which of the following is correct as the cause of stability of completely filled and Half-filled sub-shells?

• A. It is because of the symmetrical distribution of electrons
• B. It is because of relatively low shielding
• C. It is because of larger exchange energy
• D. It is because of smaller coulombic repulsion energy.

Answer 1

Answer: (A), (B), (C), (D)

It is because of smaller coulombic repulsion energy.

The completely filled and completely 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 subshell (here 3d) have equal energy but different spatial distribution. Consequently, their shielding of one-another is relatively small and the electrons are momre strongly attracted by the nucleus.

2. Exchange Energy: The stabilizing effect arises whenever two or more electrons with the same spin are present in 2. 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.
You may note that the exchange energy is at the basis of Hund’s rule that electrons which enter orbitals of equal energy have parallel spins as far as possible. In other words, 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. Details about the exchange energy will be dealt with in higher classes.