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

Which of the following has the least electron affinity in $kJ\,mol^{-1}$?

When the first electron gain enthalpy $(\Delta _{eg}H)$ of oxygen is $-141 \ kJ/mol,$ its second electron gain enthalpy is

Which of the following has the maximum electron gain enthalpy?

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

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,