Question

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

Answer 1

When an electron is added to oxygen atom to form ion, energy is released. Hence, first electron gain enthalpy of oxygen is negative.

But when another electron is added to ${O}^{-}$ion to form ${O}^{2-}$ ion, it feels stronger electrostatic repulsion. Hence, addition of second electron takes place with absorption of energy. That's why the second electron gain enthalpy of oxygen is positive.