The extra stability of half-filled subshell is due to :
(A) Symmetrical distribution of electrons
(B) Smaller coulombic repulsion energy
(C) The presence of electrons with the same spin in non-degenerate orbitals
(D) Larger exchange energy
(E) Relatively smaller shielding of electrons by one another
Identify the correct statements

The concept of “extra stability of half-filled subshells” is explained by Hund’s rule. According to this rule, when orbitals of equal energy (degenerate orbitals) are available, electrons occupy them singly with parallel spins before pairing begins. Three physical arguments support the added stability of a half-filled configuration.

1. Symmetrical distribution of charge
For a half-filled subshell, the probability cloud of electrons is distributed as uniformly as possible around the nucleus. This symmetry lowers the potential energy of the atom. Hence Statement (A) is correct.

2. Smaller coulombic repulsion and smaller shielding
When each degenerate orbital holds just one electron, there is no intra-orbital electron-electron repulsion. Inter-orbital repulsion is also reduced because the electrons tend to stay farther apart. Less repulsion means lower potential energy, and the mutual shielding of nuclear charge is also reduced. Thus both Statement (B) (“smaller coulombic repulsion energy”) and Statement (E) (“relatively smaller shielding of electrons by one another”) are correct.

3. Larger exchange energy
If there are $$n$$ electrons with parallel spins in a set of degenerate orbitals, the number of possible exchanges between them is $$\frac{n(n-1)}{2}$$. Each exchange lowers the energy slightly. A half-filled subshell maximises the number of such exchanges, giving the greatest lowering of energy. Therefore Statement (D) is correct.

4. Incorrect wording in Statement (C)
Statement (C) talks about “presence of electrons with the same spin in non-degenerate orbitals”. Hund’s rule applies to degenerate orbitals (same energy), not non-degenerate ones. So Statement (C) is incorrect.

Collecting the valid statements: (A), (B), (D) and (E).

Hence the correct option is Option B.