Question

Write Lewis structure of $O_2^{-}$ ion and find out oxidation state of each oxygen atom? What is the average oxidation state of oxygen in this ion?

Answer 1

Oxygen atom with zero charges has 6 electrons, therefore the oxidation state is 0.

And where the oxygen atom has a negative charge, it has 7 electrons. Hence, the oxidation state is -1.

The average oxidation state is -
$$\begin{array}{cc}& =\frac{0+(-1)}{2}\\ & =\frac{-1}{2}\end{array}$$