Which of the following elements have the highest and lowest electron gain enthalpy,respectively?

The electron affinity values of elements $A, B, C$ and $D$ are respectively $-135, -60, -200$ and $-348 \ kJ \ mol^{-1}$. The outer electronic configuration of element $B$ is

Which element in each of the following pairs has a more negative electron gain enthalpy?
$(i)$ $O$ or $F$ $(ii)$ $F$ or $Cl$

The set containing the elements with positive electron gain enthalpies is

Explain the periodicity of electron gain enthalpy of elements in the periodic table.

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: