Why is the addition of an electron to a uni-negative ion (except to VIA group elements) is an endothermic process whereas the formation of a uni-negative ion from its neutral atom is an exothermic process?

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Solution

Due to the strong nuclear force offered by the nucleus of the neutral atom, the attraction between the nucleus and the newly added electron is very high and hence energy is released in the process of addition of electrons to the neutral atom giving rise to an exothermic process.

When an electron is further added to a uninegative ion, there is an interelectronic repulsions offered by the extra electron to the incoming electron, and hence energy has to be supplied in order to overcome this repulsion, thus giving rise to an endothermic process

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