38. Write Lewis structure of ${{\mathrm{O}}_{2}}^{-}$ ion and find out oxidation state of each oxygen atom? What is the average oxidation state of oxygen in this ion?

The Lewis structure of ${{\mathrm{O}}_{2}}^{-}$is
Oxygen atom carrying no charge has six electrons, so its oxidation number is zero. But oxygen atom carrying - 1 charge has 7 electrons, so its oxidation number is -1.
Average oxidation number of each oxygen atom$=\frac{1}{2}$
${{\mathrm{O}}_{2}}^{-}=2\mathrm{x}=-1\phantom{\rule{0ex}{0ex}}\mathrm{x}=-\frac{1}{2}$