Question

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

O(g) + e–$\longrightarrow$ O–(g) ; $\Delta$egH = – 141 kJmol–1

O–(g) + e–$\longrightarrow$ O2–(g) ;$\Delta$egH = + 780 kJmol–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. Oxygen is more electronegative.
• B. Addition of electron in oxygen results in larger size of the ion.
• C. Electron repulsion outweighs the stability gained by achieving noble gas configuration.
• D. O– ion has comparatively smaller size than oxygen atom.

Answer 1

Answer: (C)

Electron repulsion outweighs the stability gained by achieving noble gas configuration.

Answer 2

There is electrostatic repulsion between the two species having same type of charge. So energy has to be given for the addition of additional electron to O–.