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Question
Write Lewis strucure of \[\ce{O^{-}2}\] ion and find out oxidation state of each oxygen atom? What is the average oxidation state of oxygen in this ion?
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Solution
Lweis structure of \[\ce{O^{-}2}\] is 
Oxygen atom with no charge has 6 electrons. Thus, its oxidation state is 0. Oxygen atom that carries negative charge has 7 electrons. Thus, its oxidation state is – 1.
Average O.S. = `(0 + (-1))/2 = (-1)/2`.
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