Sodium and magnesium form oxides with lower oxidation numbers, while heavier atoms form oxides with higher oxidation numbers. Sodium forms oxide and peroxide, whereas oxygen has an oxidation number of -2 and -1, respectively.
⇒ Also Read: Properties of d block elements
In magnesium oxide, oxygen is in a -2 state. But caesium forms super-oxides where the oxidation state of oxygen is – 0.5. Similarly, the heavier barium form peroxide, having an oxidation state of oxygen as -1.