Formation of `F^(-)` is exothermic but formation of `O2^(-)` is endothermic. Explain in two or three sentences.

Explanation

Formation of `F^(-)` is exothermic because incoming electron experiences force of attraction due to the nucleus of F atom. Formation of `O^(2-)` is a two step process
`O(g)+e to O^(-)(g)" "(triangle_(eg) H)_(1)=-EA_(1)`
`O^(-)(g) +e^(-) to O^(2-) (g)" "(triangle_(eq) H)_(2)=+EA_(2)`
The first one is exothermic while the second one is endothermic as energy is required to overcome the force of repulsion and `|triangle_(eg) H)(1)| lt |(triangle_(eg) H)_(2)|`