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Among the following processes, how many result in a positive (endothermic) electron gain enthalpy? (i) O -> O⁻ (ii) F -> F⁻ (iii) Na -> Na⁻ (iv) N -> N⁻ (v) N⁻ -> N²- (vi) Cl -> Cl⁻

1. 1
2. 2
3. 3
4. 4

Correct answer: 2

Solution

Processes (iii) Na -> Na⁻ and (v) N⁻ -> N²- have positive electron gain enthalpies: Na has a filled 3s subshell making anion formation unfavorable, and N²- requires forcing an electron onto an already negative ion. Processes (i), (ii), (iv), and (vi) release energy. Wait - more carefully: (i) O -> O⁻ is exothermic (negative EGE); (ii) F -> F⁻ is exothermic; (iv) N -> N⁻ is endothermic (positive EGE, due to half-filled 2p stability); (v) N⁻ -> N²- is endothermic (positive EGE, adding to anion); (iii) Na -> Na⁻ is endothermic; (vi) Cl -> Cl⁻ is exothermic. So (iii), (iv), (v) = 3 have positive EGE.

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