The formation of the oxide ion $O^{2-}_{(g)}$ requires first an exothermic and then an endothermic step as shown below:
$O_{(g)} + e^{-} \to O^{-}_{(g)}; \Delta H = -142 \, kJ \, mol^{-1}$
$O^{-}_{(g)} + e^{-} \to O^{2-}_{(g)}; \Delta H = 844 \, kJ \, mol^{-1}$
This is because:

  • A
    $O^{-}$ ion has comparatively larger size than oxygen atom
  • B
    Oxygen has high electron affinity
  • C
    $O^{-}$ ion will tend to resist the addition of another electron
  • D
    Oxygen is more electronegative

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The amount of energy released for the process $X_{(g)} + e^- \to X^-_{(g)}$ is minimum and maximum respectively for :-
$(a) \ F$
$(b) \ Cl$
$(c) \ N$
$(d) \ B$
Correct answer is :-

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In which of the following options are the elements correctly arranged with respect to their negative electron gain enthalpies?

The electron affinities of halogens are $F = 322, Cl = 349, Br = 324, I = 295 \ kJ \ mol^{-1}$. The higher value for $Cl$ as compared to that of $F$ is due to :-

The formation of the oxide ion $O_{(g)}^{2-}$ requires first an exothermic and then an endothermic step as shown below. This is because
$O_{(g)} + e^{-} \rightarrow O_{(g)}^{-}; \Delta H^{o} = -142 \ kJ \ mol^{-1}$
$O_{(g)}^{-} + e^{-} \rightarrow O_{(g)}^{2-}; \Delta H^{o} = 844 \ kJ \ mol^{-1}$

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