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

Explanation

The second electron gain enthalpy of O is positive as explained below :
When an electron is added to O atom form `O^(-)` ion, energy is released. Thus first electron gain enthalpy of O is negative
`O ( g) + e^(-) (g) to O^(-) ( g) , Delta_(eg) H = - 141 kJ mol^(-1)`
But when another electron is added to `O^(-)` to form `O^(2-)` ion energy is absorbed to overcome the strong electrostatic repulsion between the negatively charged `O^(-)` ion and the second electron being added Thus the second electron gain enthalpy of oxygen is positive ,
`O^(-) ( g) + e^(-) (g) to O^(2-) ( g) , Delta_(eg) H = + 780 kJ mol^(-1)`