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

Concept: Electron Gain Enthalpy and Electrostatic Repulsion

The first electron gain enthalpy of oxygen is negative (exothermic) because a neutral O atom readily accepts an electron into its valence shell to form O⁻, moving closer to a stable octet:

The second electron gain enthalpy involves adding an electron to an already negatively charged O⁻ ion:

$\text{O}^-(g) + e^- \to \text{O}^{2-}(g) \quad \Delta_{\text{eg}}H_2 = ?$ …

Why electron gain enthalpy changes sign

Electron gain enthalpy measures the energy change when an isolated gaseous atom gains an electron. The first electron gain enthalpy of oxygen is negative—the process releases energy—because the incoming electron experiences the strong nuclear attraction of 8 protons and completes a more stable electronic configuration.

But what happens when we try to add a second electron to form O²⁻? Now we're not dealing with a neutral atom anymore. We're pushing an electron into an ion that already carries a full negative charge.

The electrostatic reality

1. First electron addition: O(g) → O⁻(g)

   The neutral oxygen atom has 8 protons in the nucleus attracting the incoming electron. Yes, there's some repulsion from the existing 8 electrons, but the net effect is attractive. The electron configuration goes from $1s^2 \, 2s^2 \, 2p^4$ to $1s^2 \, 2s^2 \, 2p^5$, moving closer to the stable noble gas configuration. Energy is released: $\Delta_{eg}H_1 = -141 \, \text{kJ mol}^{-1}$.

2. Second electron addition: O⁻(g) → O²⁻(g)

   Now the species already has a net negative charge. The incoming electron faces:

   • Attraction from the 8 protons (still there, unchanged)
   • Strong repulsion from the 9 electrons already present, especially the extra electron that made O⁻ negative

   The repulsion dominates. You're essentially trying to compress two like charges together. This requires an input of energy—the process is endothermic.

3. The energy cost …