Would you expect the second electron gain enthalpy of O as positive , more negative or less negative than the first? Justify your answer.

Explanation

When an electron is added to `O` atom to form `O^(ө)` ion, energy is released. Thus, the first electron gain enthalpy of `O` is negative.
`O_((g))+e_((g))^(ө)rarrO_((g))^(ө)`,`Delta_(eg)H^(ө)=-141 kJ mol^(-1)`
But when another electron is added to `O^(ө)` to form `O^(2-)` ion, energy is absorbed to overcome the strong electrostatic repulsion between the negatively charged `O^(ө)` ion and the second electron being added. Thus,the second electron gain enthalpy of oxygen is positive.
`O_((g))^(ө)+e_((g))^(ө)rarrO_((g))^(2-)`,`Delta_(eg)H^(ө)=+780 kJ "mol"^(-1)`