After addition of one electron to the oxygen atom, it becomes negatively charged and the second electron is added to a negatively charged ion. The addition of second electron is opposed by the columbic force of repulsion and energy has to be supplied for the addition of the second electron since the oxygen atom is smaller in size, the electron-electron repulsions in the small 2p-subshell are comparatively large and hence, it is forced to accept the second electron. Consequently, its second electron gain enthalpy is positive.
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