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

Explanation

For oxygen atom:
`O_((g))+e^(-)toO_((g))^(-)`(e.g `DeltaH=-141KJ mol^(-1)`)
`O_((g))^(-)+e^(-)toO_((g))^(2-)` (e.g `DeltaH=+780 KJ mol^(-1))`
The first electron gain enthalpy of oxygen is negative because energy is released when a gaseous atom accepts an electron to form monovalent anion. The second electron gain enthalpy is positive because energy is needed to overcome the force of repulsion between monovalent anion and second incoming electron.