The formation of the oxide ion, O 2 - ( g ) , from oxygen atom requires first an exothermic and then an endothermic step as shown in below: O g + e - ⟶ O - g ; Δ f H ⊖ = - 141 k J m o l - 1 O - g + e - ⟶ O 2 - g ; Δ f H ⊖ = + 780 k J m o l - 1 Thus process of formation of O 2 - in gas phase is unfavourable though O 2 - is isoelectronic with neon. It is due to the fact that,

Question

The formation of the oxide ion, O 2 - ( g ) , from oxygen atom requires first an exothermic and then an endothermic step as shown in below: O g + e - ⟶ O - g ; Δ f H ⊖ = - 141 k J m o l - 1 O - g + e - ⟶ O 2 - g ; Δ f H ⊖ = + 780 k J m o l - 1 Thus process of formation of O 2 - in gas phase is unfavourable though O 2 - is isoelectronic with neon. It is due to the fact that,, Electron repulsion outweighs the stability gained by achieving noble gas configuration., 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.

MARKS App · Accepted Answer

Due to the fact that electron repulsion outweighs the stability gained by achieving noble gas configuration, formation of O 2 - in gas phase is unfavourable though O 2 - 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.

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