The electron affinity for the inert gases is

Within each pair of elements of $F$ and $Cl$,$S$ and $Se$,and $Li$ and $Na$,respectively,the elements that release more energy upon an electron gain are

The set containing the elements with positive electron gain enthalpies is

The formation of the oxide ion $O^{2-}_{(g)}$ requires first an exothermic and then an endothermic step as shown below:
$O_{(g)} + e^{-} \to O^{-}_{(g)}; \Delta H = -142 \, kJ \, mol^{-1}$
$O^{-}_{(g)} + e^{-} \to O^{2-}_{(g)}; \Delta H = 844 \, kJ \, mol^{-1}$
This is because:

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

Arrange $N$,$O$,and $S$ in order of decreasing electron affinity.