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Question
Explain the following, give an appropriate reason.
The formation of \[\ce{F^-_{(g)}}\] from \[\ce{F_{(g)}}\] is exothermic while that of \[\ce{O^2-_{(g)}}\] from \[\ce{O_{(g)}}\] is endothermic.
Solution
\[\ce{F_{(g)} + e^- -> F^-_{(g)}}\] exothermic
F (Z = 9) 1s2 2s2 2p5.
It is ready to gain one electron to attain the nearest inert gas configuration. By gaining one electron, energy is released, so it is an exothermic reaction.
\[\ce{O_{(g)} + 2e^- -> O^2-_{(g)}}\] exothermic
O (Z = 8) 1s2 2s2 2px1 2py1 2pz1.
It is a small atom with a high electron density. The first electron affinity is negative because energy is released in the process of adding one electron to the neutral oxygen atom. Second electron affinity is always endothermic (positive) because the electron is added to an ion that is already negative, therefore it must overcome the repulsion.
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