Question

The formation of the oxide ion $O^{2-}$ involves first an exothermic step followed by an endothermic step as shown below:
$O+e^{-}\to O^{-};\Delta H=-142kJmo{l}^{-1}$

$O^{-}+e^{-}\to O^{2-};\Delta H=844kJmo{l}^{-1}$

This is because :

• A. $O^{-}$ ion has comparatively larger size than oxygen atom
• B. oxygen has high electron affinity
• C. $O^{-}$ ion will tend to resist the addition of another electron.
• D. oxygen is more electronegative

Answer 1

The correct option is C $O^{-}$ ion will tend to resist the addition of another electron.

After adding one electron, electron repulsion would be very high. Hence, to add one more electron we are supposed to give energy to the system, hence endothermic in the second case.