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.

Answer 1

\[\ce{F_{(g)} + e^- -> F^-_{(g)}}\] 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 2p_x¹ 2p_y¹ 2p_z¹.

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.