The formation of the oxide ion, , requires an initial exothermic step followed by an endothermic step. Why is the second step endothermic despite achieving a noble gas configuration?

Question

The formation of the oxide ion, $O^{2-}_{(g)}$, requires an initial exothermic step followed by an endothermic step. Why is the second step endothermic despite achieving a noble gas configuration?

Prashant Kotian · Accepted Answer

Correct answer: B. Adding an electron to a neutral oxygen atom releases energy. However, forcing a second negative electron onto an already negative ion causes immense electrostatic repulsion, requiring a large input of energy ().

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