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); H = -141 kJ mol-1 O-(g) + e- O2-(g); H = +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,
- O- ion has comparatively smaller size than oxygen atom
- oxygen is more electronegative
- addition of electron in oxygen results in larger size of the ion
- electron repulsion outweighs the stability gained by achieving noble gas configuration.
Answer
(4) electron repulsion outweighs the stability gained by achieving noble gas configuration.
Solution
The second electron has to be added to the negatively charged O- ion.
⇒ The incoming electron is strongly repelled by the negative charge (and by the electrons already crowded in the small 2p subshell), so energy must be supplied: H = +780 kJ mol-1.
Net enthalpy = -141 + 780 = +639 kJ mol-1 (endothermic)
∴ The electron–electron repulsion outweighs the stability gained by attaining the neon configuration.
Answer: (4)