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The formation of the oxide ion O2−(g), from oxygen atom requires first an exothermic and then an endothermic step as shown below: O(g) + e− → O−(g); ΔH° = −141 kJ mol−1 O−(g) + e− → O2−(g); ΔH° = +780 kJ mol−1 Thus process of formation of O2− in gas phase is unfavourable even though O2− is isoelectronic with neon. It is due to the fact that:

1. Electron repulsion outweighs the stability gained by achieving noble gas configuration
2. O− ion has comparatively smaller size than oxygen atom
3. Oxygen is more electronegative
4. Addition of electron in oxygen results in larger size of the ion

Correct answer: Electron repulsion outweighs the stability gained by achieving noble gas configuration

Solution

The second electron addition to form O2− is endothermic because the strong electron-electron repulsion in the already negatively charged O− ion outweighs the stability gained by achieving a noble gas configuration.

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