What is the value of electron gain enthalpy of $Na^{+}$ if $IE_{1}$ of $Na$ is $5.1 \ eV$? .......... $eV$

Which of the following represents the correct order of electron gain enthalpy (electron affinity) for halogens?

Consider the following conversions:
$(i) \ O_{(g)} + e^- \to O^{-}_{(g)} ; \Delta H_1$
$(ii) \ F_{(g)} + e^- \to F^{-}_{(g)} ; \Delta H_2$
$(iii) \ Cl_{(g)} + e^- \to Cl^{-}_{(g)} ; \Delta H_3$
$(iv) \ Na_{(g)} \to Na^{+}_{(g)} + e^- ; \Delta H_4$
Which of the following statements is incorrect?

Assertion $(A)$: Fluorine has a smaller negative electron gain enthalpy than chlorine.
Reason $(R)$: The electron-electron repulsion is higher in chlorine than in fluorine.

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,