The formation of the oxide ion $O^{2-}_{(g)}$ requires first an exothermic and then an endothermic step as shown below:
$O_{(g)} + e^{-} \to O^{-}_{(g)}; \Delta H = -142 \, kJ \, mol^{-1}$
$O^{-}_{(g)} + e^{-} \to O^{2-}_{(g)}; \Delta H = 844 \, kJ \, mol^{-1}$
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

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