Chemistry · Classification of Elements and Periodicity in Properties
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) ; ΔHΘ = -141 kJ mol-1 O- (g) + e- → O2- (g); ΔHΘ = +780 kJ mol-1 Thus, the process of formation of O2- in the gas phase is unfavourable even though O2- is isoelectronic with neon. It is due to the fact that:
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- 1O- ion has comparatively smaller size than oxygen atom.
- 2electron repulsion outweighs the stability gained by achieving noble gas configuration.✓
- 3addition of electron in oxygen results in larger size of the ion.
- 4oxygen is more electronegative.
Correct answer
electron repulsion outweighs the stability gained by achieving noble gas configuration.
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