The Total solution for NCERT class 6-12
When an electron is added to O atom to form O– ion, energy is released. Thus, the firstelectron gain enthalpy of O is negative.
On the other hand, when an electron is added to O– ion to form O2– ion,energy has to be given out in order to overcome the strong electronicrepulsions. Thus, the second electron gain enthalpy of O is positive.