Question

The formula of the oxide ion $O^{2-}\left(g\right)$ requires first an exothermic and then an endothermic step as shown below:

$O\left(g\right)+e^{-}\to O^{-}\left(g\right);△H^o=-142kJ{mol}^{-1}$

$O^{-}\left(g\right)+e^{-}\to O^{2-}\left(g\right);△H^o=844kJ{mol}^{-1}$.

The correct statement is:

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

Answer 1

The correct option is A oxygen is more electronegative

$O\left(g\right)+e^{-}\to O^{-}\left(g\right)$....(i) $△H^o=-192kJ/mol$

$O^{-}\left(g\right)+e^{-}\to O^{2-}\left(g\right)$....(ii) $△H^o=844kJ/mol$

In reaction (i), since oxygen is electronegative and also have incomplete shell. Hence, gain one electron and energy is released but in 2nd reaction oxygen has all ready negative charge and addition of one more electron require energy due to repulsion. Hence, $△H$ is positive.