The order of electron affinity for $F, Cl, Br,$ and $I$ is .....

Which of the following is correct for the electron affinity of halogens?

Which halogen has the highest value of negative electron gain enthalpy?

The least stable ion is

Discuss the factors affecting electron gain enthalpy and the trend in its variation in the periodic table.

The formation of oxide ion $O^{2-}$ from oxygen atom requires an exothermic reaction,followed by an endothermic step as shown below. The process of formation of $O^{2-}$ in gas phase is unfavorable $(\Delta H^{\ominus} = +ve)$,even though it has the stable configuration of the nearest noble gas neon. This is because:
$O_{(g)} + e^{-} \longrightarrow O^{-}_{(g)} ; \Delta H^{\ominus} = -141 \ kJ \ mol^{-1}$
$O^{-}_{(g)} + e^{-} \longrightarrow O^{2-}_{(g)} ; \Delta H^{\ominus} = +760 \ kJ \ mol^{-1}$