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Which of the following processes is exothermic?

1. N⁻ -> N + e⁻
2. Na -> Na⁺ + e⁻
3. O⁻ + e⁻ -> O²-
4. Ca⁺ -> Ca²+ + e⁻

Correct answer: N⁻ -> N + e⁻

Solution

Electron affinities: the second electron affinity (adding e⁻ to O⁻) is always endothermic because of electron-electron repulsion. Ionisation energies (Na->Na⁺, Ca⁺->Ca²+) always require energy input (endothermic). N⁻ -> N + e⁻ is exothermic because N⁻ is unstable relative to neutral N (nitrogen's half-filled 2p is especially stable). The reverse process (N + e⁻ -> N⁻) has a negative electron affinity, meaning it is endothermic; so the forward process releases energy.

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