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.
$O_{(g)}+e^-$ $\rightarrow O^-_{(g)}$
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.
$O^-_{(g)}+e^- \rightarrow O^{2-}_{(g)}$