Would you expect the second electron gain enthalpy of O as positive, more negative or less negative than the first? Justify your answer.
The second electron gain enthalpy of O is expected to be positive.
When the first electron is added to a neutral oxygen atom, energy is released. However, when a second electron is added to the negatively charged ion, it experiences strong repulsion from the existing negative charge and the other electrons present in the smaller quantum level.
Because of this significant repulsion, energy must be supplied to force the second electron into the unstable negatively charged ion. Therefore, the second electron gain enthalpy is positive, unlike the first which is negative.
Explanation
The first electron gain enthalpy of oxygen is negative because the added electron is attracted to the neutral atom. However, adding a second electron to the already negatively charged O- ion is difficult. The context highlights that adding an electron to the smaller n=2 level of O causes significant repulsion. When adding a second electron to an anion, this repulsion is even greater, requiring energy input rather than releasing it, which makes the second electron gain enthalpy positive.