Electron gain enthalpy (Δ_egH) is the energy change when an atom gains an electron to form a negative ion. For oxygen (atomic number 8), the electronic configuration is $1s^{2}2s^{2}2p^4$.

Step 1: First Electron Gain Enthalpy
When oxygen gains the first electron, it becomes O^–. This process is exothermic, releasing energy, so Δ_egH is negative (-141 kJ/mol in this case). The added electron enters the partially filled 2p subshell, which is energetically favorable.

Step 2: Second Electron Gain Enthalpy
Adding a second electron to O^– to form O^2– requires forcing an electron into an already negatively charged ion. This involves significant electron-electron repulsion. Therefore, energy must be absorbed to overcome this repulsion, making the second electron gain enthalpy positive.

Final Answer: The second electron gain enthalpy of oxygen is a positive value.

Related Topics & Formulae

Electron Gain Enthalpy Trend: Generally becomes less negative across a period and more negative down a group. Elements with half-filled or fully filled orbitals have less negative (or even positive) values.

Key Theory: The sign and magnitude of electron gain enthalpy depend on atomic size, nuclear charge, and electronic configuration. Adding an electron to a neutral atom is often exothermic, but adding to a negative ion is endothermic due to repulsion.