The formation of the oxide ion, O^{2-}(g), from oxygen atom requires first an exothermic and then an endothermic step as shown below: Thus process of formation of O^{2-}  in gas phase is unfavourable even though O^{2-}  is isoelectronic with neon. It is due to the fact that,

O(g)+e^{-}\rightarrow O^{-}(g);\Delta H^{\ominus }= - 141 kJ\; mol^{-1}

O^{-}(g)+e^{-}\rightarrow O^{2-}(g);\Delta H^{\ominus }= +780\; kJ\; mol^{-1}

(i) oxygen is more electronegative.

(ii) addition of electron in oxygen results in larger size of the ion.

(iii) electron repulsion outweighs the stability gained by achieving noble gas configuration.

(iv) O ion has comparatively smaller size than oxygen atom.

Answers (1)

The answer is the option (iii) electron repulsion outweighs the stability gained by achieving noble gas configuration.

Explanation:  Energy is released whenever an electron is added to the O atom to form O-  ion. Thus the first electron gain enthalpy of O is negative. Contrary to this whenever an electron is added to O- ion to form O2- ion, energy has to be given out in order to overcome the very strong electronic repulsions, and that is why the second gain enthalpy of O is positive.

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