The formation of the oxide ion `O_((g))^(2-)` requires first an exothermic and then an endothermic step as shown below.
`O_((g)) +e^(-) = O_((g))^(-) DeltaH^(@) =- 142 kJ mol^(-1)`
`O_((g))^(-) + e^(-) = O_((g))^(2-) DeltaH^(@) = 844 kJ mol^(-1)` This is because of :