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NEET 2015
Exam
NEET
Year
2015
Subject
Chemistry
Chapter
Classification of Elements and Periodicity in Properties

NEET 2015 · Chemistry · Classification of Elements and Periodicity in Properties

The formation of the oxide ion, O ^ 2 - ( g ) , from oxygen atom...

The formation of the oxide ion, O2−(g){O}^{2 -} ( 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\left(g\right) + {e}^{-} ⟶ {O}^{-}\left(g\right); {\Delta}_{f}{H}^{⊖} = - 141kJmo{l}^{- 1}

O−(g)+e−⟶O2−(g);ΔfH⊖=+780kJmol−1{O}^{-}\left(g\right) + {e}^{-} ⟶ {O}^{2 -}\left(g\right); {\Delta}_{f}{H}^{⊖} = + 780kJmo{l}^{- 1}

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

Options

  1. AElectron repulsion outweighs the stability gained by achieving noble gas configuration.
  2. BO−{O}^{-} Ion has comparatively smaller size than oxygen atom.
  3. COxygen is more electronegative.
  4. 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 O2−{O}^{2 -} in gas phase is unfavourable though O2−{O}^{2 -} is isoelectronic with neon. When an electron is added to O−{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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