Electron Gain Enthalpy Questions

Which one of the following arrangements represents the correct order of electron gain enthalpy (with negative sign) of the given atomic species ?

Which of the following represents the correct order of increasing electron gain enthalpy with negative sign for the elements $\text{O, S, F and Cl}$?

What is the value of electron gain enthalpy of ${\text{Na}}^{+}$ if $I{E}_1$ of $\text{Na}=5.1\text{eV}$?

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