Classification of Elements and Periodicity in Properties - Result Question 13
####13. The formation of the oxide ion $O^{2-}(g)$, from oxygen atom requires first an exothermic and then an endothermic step as shown below : $O(g)+e^{-} \to O^{-}(g) ; \Delta_f H^{\ominus}=-141 kJ mol^{-1}$ $O^{-}(g)+e^{-} \to O^{2-}(g) ; \Delta_f H^{\ominus}=+780 kJ mol^{-1}$ 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 [2015 RS]
(a) Electron repulsion outweighs the stability gained by achieving noble gas configuration
(b) $O^{-}$ion has comparatively smaller size than oxygen atom
(c) Oxygen is more electronegative
(d) Addition of electron in oxygen results in larger size of the ion.
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Solution:
- (a) Incoming electrons occupies the smaller $n$ $=2$ shell, also negative charge on oxygen $(O^{-})$is another factor due to which incoming electron feel repulsion.
Hence, electron repulsion outweigh the stability gained by achieving noble gas configuration.
