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Match each element in List-I with its first ionisation enthalpy (delta H_IE1 in kJ/mol) in List-II. List-I: (P) N, (Q) O, (R) Be, (S) B List-II: (1) +800, (2) +900, (3) +1300, (4) +1400

1. P -> 2; Q -> 3; R -> 1; S -> 4
2. P -> 1; Q -> 2; R -> 3; S -> 4
3. P -> 4; Q -> 3; R -> 2; S -> 1
4. P -> 1; Q -> 4; R -> 2; S -> 3

Correct answer: P -> 4; Q -> 3; R -> 2; S -> 1

Solution

Nitrogen has a half-filled 2p³ configuration (extra stability) giving it a higher IE1 (~1402 kJ/mol, matched to 1400). Oxygen has IE1 ~1314 kJ/mol (matched to 1300) — lower than N because one electron must be paired in 2p, increasing repulsion. Beryllium has IE1 ~899 kJ/mol (matched to 900) due to its stable full 2s² subshell. Boron has the lowest IE1 here (~800 kJ/mol) because it loses a 2p electron, which is shielded by the 2s electrons.

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