Concept:Half-filled subshells (like Nitrogen's \( 2p^3 \)) provide extra stability. Adding one more electron creates pairing repulsion, making it easier to remove.
Formula:$$ \text{N: } 2p^3 \text{ (stable)} \quad vs \quad \text{O: } 2p^4 \text{ (repulsion)} $$
Solution:- Nitrogen has a perfectly half-filled p-subshell, which is symmetrically stable.
- Oxygen has four p-electrons, meaning one of the p-orbitals must contain a pair of electrons.
- This pairing causes inter-electronic repulsion, destabilizing the electron and making it easier to remove from Oxygen than from the highly stable Nitrogen atom.
Why other options are incorrect:- Option C: A greater nuclear charge would logically increase ionization energy. The anomaly is purely due to electron repulsion.
- Option B & Option D: True statements about Oxygen, but they do not explain the ionization energy anomaly.
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