Extra Stability of Half Filled Subshells Explained

 

❓ Question

The extra stability of half-filled subshells is due to which of the following factors?

Options:
(A) Symmetrical distribution of electrons
(B) Smaller Coulombic repulsion energy
(C) Presence of electrons with same spin in non-degenerate orbitals
(D) Larger exchange energy
(E) Relatively smaller shielding of electrons by one another

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🖼️ Question Image

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✍️ Short Explanation

This problem is based on:

👉 Electronic configuration
👉 Exchange energy
👉 Stability of half-filled orbitals.

Main idea:

Half-filled and fully-filled subshells are extra stable mainly because of:

✔ Symmetrical distribution of electrons
✔ Maximum exchange energy

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🔷 Step 1 — Symmetrical Distribution 💯

In half-filled subshells:

Example:

$$p^3,\ d^5,\ f^7$$

electrons distribute uniformly in all orbitals.

This gives:

✔ Better symmetry
✔ Lower energy
✔ Greater stability

Thus:

$$\boxed{(A)\ \text{is correct}}$$

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🔷 Step 2 — Exchange Energy

Electrons with parallel spins can exchange positions.

More number of exchanges:

$$\Rightarrow \text{More exchange energy}$$

More exchange energy means:

$$\Rightarrow \text{Greater stability}$$

Half-filled subshell has maximum possible exchange energy.

Thus:

$$\boxed{(D)\ \text{is correct}}$$

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🔷 Step 3 — Check Other Statements

(B) Smaller coulombic repulsion

Not the main accepted reason.

❌ Incorrect.

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(C) Same spin in non-degenerate orbitals

Orbitals are actually degenerate, not non-degenerate.

❌ Incorrect statement.

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(E) Smaller shielding

Not responsible for half-filled stability.

❌ Incorrect.

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🔷 Step 4 — Final Conclusion

Correct reasons are:

$$\boxed{ (A)\ \text{and}\ (D) }$$

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🔷 Step 5 — JEE Trap Alert 🚨

❌ Degenerate vs non-degenerate orbitals confuse kar dena

❌ Exchange energy concept bhool jaana

Remember:

Half-filled stability mainly due to:

$$\boxed{ \text{Symmetry + Exchange Energy} }$$

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✅ Final Answer

$$\boxed{ (A)\ \text{and}\ (D) }$$

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