Question 135 of 209

The formation of the oxide ion O2-(g), from oxygen atom requires first an exothermic and then an endothermic step as shown

O(g) + e-           O-(g); fH° = -141kJmol-1O-(g) +e-           O2-(g); fH° = +780 kJ mlo-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
(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 result in large size of the ion