The formation of oxide ion O ²⁻ from oxygen atom requires an exothermic reaction, followed by an endothermic step as shown below. The process of formation of O ²⁻ in gas phase is unfavorable ( H^ =+ . ve ) , even though it has stable configuration of the nearest noble gas neon. This is because aligned & O (g)+e⁻ O ⁻(g) ; H^ =-141 ~kJ ~mol ⁻¹ & O ⁻(g)+e⁻ O ²⁻(g) ; H^ =+760 ~kJ ~mol ⁻¹ aligned

Step-by-step solution

The process of formation of O ²⁻ in the gas phase is unfavorable even through O ²⁻ is isoelectronic with neon. It is due to that electronic repulsion outweighs the stability gained by achieving noble gas configuration. The formation of oxide ion, O ²⁻(g) from oxygen atom require first an exothermic and then endothermic step. aligned & O (g)+e⁻ O ⁻(g) ; H^ =-141 ~kJ ~mol ⁻¹ & O ⁻(g)+e⁻ O ²⁻(g) ; H^ =+760 ~kJ ~mol ⁻¹ aligned When an electron is added to O ⁻ anion, there is strong electrostatic repulsion between two n