Concept: why half-filled subshells are extra stable

Half-filled and completely filled subshells show extra stability mainly due to:

• symmetrical distribution of electrons,
• maximum exchange energy,
• and relatively lower electron-electron repulsion in such arrangements.

Examine each statement

(A) Symmetrical distribution of electrons

• In a half-filled subshell, electrons occupy all orbitals singly.
• This gives a symmetrical arrangement, which is more stable.
• Hence, (A) is correct.

(B) Smaller coulombic repulsion energy

• In a half-filled subshell, electrons remain unpaired in different orbitals before pairing occurs.
• This reduces electron-electron repulsion compared to paired arrangements.
• Hence, (B) is correct.

(C) The presence of electrons with the same spin in non-degenerate orbitals

• Exchange energy arises when electrons with parallel spin are present in degenerate orbitals, not non-degenerate orbitals.
• Since the statement says non-degenerate orbitals, it is incorrect.
• Hence, (C) is incorrect.

(D) Larger exchange energy

• A half-filled subshell has the maximum number of possible exchanges among electrons with parallel spins.
• Greater exchange energy leads to greater stability.
• Hence, (D) is correct.

(E) Relatively smaller shielding of electrons by one another

• In standard explanation for extra stability of half-filled subshells, this is also considered a contributing factor.
• Hence, (E) is correct.

Collect correct statements

Correct statements are: $(A), (B), (D), (E)$

Match with the given options

This corresponds to Option B.

Comparison with stored answer

Stored correct answer: B

Our derived answer: B

So, the answers agree.