Classification of Elements and Periodicity in Properties: JEE Main 2004 Chemistry question

The formation of the oxide ion O²⁻(g) requires first an exothermic and then an endothermic step as shown below O_(g) + e⁻ = $O_{(g)}^{-}$ $\Delta$H^o = -142 kJmol⁻¹ $O_{(g)}^{-}$ + e⁻ = $O_{(g)}^{2-}$ $\Delta$H^o = 844 kJmol⁻¹ This because