# Question #95391

Since potassium forms only one ion, ${\text{K}}^{+}$, no Roman numeral is used. Oxygen forms two ions; the oxide ion, ${\text{O}}^{2 -}$, and the peroxide ion, ${\text{O"_2}}^{2 -}$.
The formula unit for potassium oxide is $\text{K"_2"O}$, which is also the empirical formula. The formula unit for potassium peroxide is ${\text{K"_2"O}}_{2}$. The ratio of $\text{K"^(+)}$ ions to ${\text{O"_2}}^{2 -}$ ions is $\text{1:1}$, so $\text{KO}$ is the empirical formula for potassium peroxide.
The reason that the formula unit for potassium peroxide is not reduced to $\text{KO}$, is that the peroxide ion has the formula ${\text{O"_2}}^{2 -}$. It is composed of two covalently bonded oxygen atoms plus two extra electrons. Therefore, the peroxide ion cannot be further reduced.