Electron gain enthalpy value for oxygen is negative $(-142kJ$ mol $e^{-1})$ while sum of electron gain enthaplies of O and $\left(O^{-}\right)$ respectively si positive is $\left(702kJ{\text{mol}}^{-1}\right)$ .Explain the reason for opposite sign for two values . Also calcularte $E{A}_2$ value .

Solution: The two changes are :
$O{+}_{-1}{e}^0\to O^{-},E_{\text{ge}}$ of $O=-142kJ{\text{mol}}^{-1}$
$O+2_{-1}{e}^0\to O^{2-},$
$E_{\text{ge}}$ of $O+E_{\text{ga}}$ of $O^{-}=702kJ{\text{mol}}^{-1}$
When an electron is added to isolated gaseous oxygen atmos energy is released because electron is attracted by nucleus but when second electron is added then uninegative ion repels it and energy is required to insert it in balance hell .
Also $E_{\text{ga}}$ of $O+E_{\text{ga}}$ of $O^{-}=702$
or $E_{\text{ga}}$ of $O^{-}=702(E_{\text{ga}}$ if $O$
$=702-\left(-142\right)-844kj\text{mol}{e}^{-1}$.