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Which of the following conversions is exothermic?

1. Na -> Na+
2. Li -> Li-
3. N -> N-
4. Mg -> Mg-

Correct answer: Li -> Li-

Solution

Na->Na+ is ionisation (endothermic). Li->Li- involves addition of an electron to Li; lithium has a positive electron affinity (~60 kJ/mol), so this process releases energy and is exothermic. N->N- is endothermic because N has a stable half-filled 2p³ configuration. Mg->Mg- is endothermic since the extra electron must enter a higher-energy 3p orbital.

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