The formation of the oxide ion, O 2 - ( g ) , from oxygen atom requires first an exothermic and then an…

The formation of the oxide ion, O2-(g) , from oxygen atom requires first an exothermic and then an endothermic step as shown in below:
Og+e-⟶O-g   ;  ΔfH⊖=-141 kJ mol-1
O-g+e-⟶O2- g ;  ΔfH⊖=+780 kJ mol-1
Thus process of formation of O2- in gas phase is unfavourable though O2- is isoelectronic with neon. It is due to the fact that,
  1. Electron repulsion outweighs the stability gained by achieving noble gas configuration.
  2. O- Ion has comparatively smaller size than oxygen atom.
  3. Oxygen is more electronegative.
  4. Addition of electron in oxygen results in larger size of the ion.

Solution

Due to the fact that electron repulsion outweighs the stability gained by achieving noble gas configuration, formation of O2- in gas phase is unfavourable though O2- is isoelectronic with neon. When an electron is added to O- anion, there is strong electrostatic repulsion between the two negative charges. Due to this, the second electron gain enthalpy of oxygen is positive.

Asked in: NEET 2015 (Phase 2)

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