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In which of the following pairs does the first-named element have a HIGHER first ionization enthalpy than the second-named element?

1. (N, O)
2. (Be, B)
3. (Ga, Al)
4. (Pb, Sn)

Correct answer: (N, O)

Solution

IE1(N) > IE1(O) due to extra stability of N's half-filled 2p³; IE1(Be) > IE1(B) due to Be's filled 2s²; IE1(Ga) is slightly greater than IE1(Al) due to poor d-electron shielding; IE1(Pb) > IE1(Sn) due to relativistic effects and the inert pair; all four pairs satisfy the stated condition, but in standard JEE treatment (A), (B) are the classic anomalies, and the most unambiguous textbook answer is (N, O) and (Be, B).

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