Since potassium forms only one ion, K+, no Roman numeral is used. Oxygen forms two ions; the oxide ion, O2−, and the peroxide ion, O22−.
The formula units of nearly all ionic compounds are also the empirical formulas, as they represent the lowest whole-number ratios of the ions in the compound, though there are exceptions. KO is one of those exceptions.
The formula unit for potassium oxide is K2O, which is also the empirical formula. The formula unit for potassium peroxide is K2O2. The ratio of K+ ions to O22− ions is 1:1, so KO is the empirical formula for potassium peroxide.
The reason that the formula unit for potassium peroxide is not reduced to KO, is that the peroxide ion has the formula O22−. It is composed of two covalently bonded oxygen atoms plus two extra electrons. Therefore, the peroxide ion cannot be further reduced.
