nursing AIIMS BSc Nursing 2025 Mock Test Series Chemistry Classification of Elements and Periodicity in Properties Periodic Table
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) ; ∆ HΘ = – 141 kJ mol-1
O- (g) + e- → O2- (g); ∆ HΘ = + 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
oxygen is more electronegative.
2
addition of electron in oxygen results in larger size of the ion.
3
electron repulsion outweighs the stability gained by achieving noble gas configuration.
4
O- ion has comparatively smaller size than oxygen atom.