NEET2015ChemistryClassification of Elements and Periodicity in PropertiesActual
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 i
Options
- AElectron repulsion outweighs the stability gained by achieving noble gas configuration.
- BO - Ion has comparatively smaller size than oxygen atom.
- COxygen is more electronegative.
- DAddition of electron in oxygen results in larger size of the ion.
Correct answer
A. Electron repulsion outweighs the stability gained by achieving noble gas configuration.
Step-by-step solution
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.