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

When a chloride ion is formed from an isolated gaseous chlorine atom,$3.8 \ eV$ of energy is released. This value is equal to the:

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:
$O_{(g)} + e^{-} \longrightarrow O^{-}_{(g)} ; \Delta H^{\ominus} = -141 \ kJ \ mol^{-1}$
$O^{-}_{(g)} + e^{-} \longrightarrow O^{2-}_{(g)} ; \Delta H^{\ominus} = +760 \ kJ \ mol^{-1}$

Explain the periodicity of electron gain enthalpy of elements in the periodic table.

The value of $2^{nd}$ electron gain enthalpy for $O$ and $S$ will be (in $kJ/mol$)

$Na^{-} \xleftarrow{x} Na \xrightarrow{5 \ eV} Na^{+}$
Predict the value of $X$ in $eV$.