For the reaction $Cl_{(g)} + e^- \to Cl^-_{(g)}$,the value of $\Delta H$ 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 correct pair of electron affinity order is

What is the correct order of electron affinity for the elements of the oxygen group in the periodic table?

The formation of the oxide ion,$O^{2-}_{(g)}$,from oxygen atom requires first an exothermic and then an endothermic step as shown below:
$O_{(g)} + e^- \to O^{-}_{(g)} ; \Delta_f H^{\Theta} = -141 \ kJ \ mol^{-1}$
$O^{-}_{(g)} + e^- \to O^{2-}_{(g)} ; \Delta_f H^{\Theta} = +780 \ kJ \ mol^{-1}$
Thus,the process of formation of $O^{2-}$ in gas phase is unfavourable even though $O^{2-}$ is isoelectronic with neon. It is due to the fact that,

Among the following configurations,the element which has the highest electron affinity is