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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:…

NEET UG 2015 — Chemistry Inorganic Chemistry

2015mcqmedium

The formation of the oxide ion, O2−(g){O}^{2-}(g)O2−(g) , from oxygen atom requires first an exothermic and then an endothermic step as shown in below:

O(g)+e−⟶O−(g);ΔfH⊖=−141kJmol−1O(g)+{e}^{-}\longrightarrow {O}^{-}(g) ; {\Delta }_{f}{H}^{\ominus }=-141 kJ mo{l}^{-1}O(g)+e−⟶O−(g);Δf​H⊖=−141kJmol−1

O−(g)+e−⟶O2−(g);ΔfH⊖=+780kJmol−1{O}^{-}(g)+{e}^{-}\longrightarrow {O}^{2-} (g) ; {\Delta }_{f}{H}^{\ominus }=+780 kJ mo{l}^{-1}O−(g)+e−⟶O2−(g);Δf​H⊖=+780kJmol−1

Thus process of formation of O2−{O}^{2-}O2− in gas phase is unfavourable though O2−{O}^{2-}O2− is isoelectronic with neon. It is due to the fact that,

Official previous-year question

Held on 30 Apr 2015 · Verified 9 Jul 2026.

Options

  1. A

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

  2. B

    O−{O}^{-}O− Ion has comparatively smaller size than oxygen atom.

  3. C

    Oxygen is more electronegative.

  4. D

    Addition of electron in oxygen results in larger size of the ion.

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