Question

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

Answer 1

When an electron is added to O atom to form O– ion, energy is released. Thus, the first electron gain enthalpy of O is negative.
`O_((g))+e^(+) rarr O_(g)^(-)`
On the other hand, when an electron is added to `O^(-)` ion to form `O^(2-)` ion, energy has to be given out in order to overcome the strong electronic repulsions. Thus, the second electron gain enthalpy of O is positive
`O_((g))^(-) +e^(-)rarrn O_((g))^(2-)`