Why do some metals form peroxides on heating in oxygen?
Beryllium, magnesium and calcium don’t form peroxides when heated in oxygen, but strontium and barium do. There is an increase in the tendency to form the peroxide as you go down the Group. The peroxide ion, O22- looks llike this: The covalent bond between the two oxygen atoms is relatively weak. Now imagine bringing a small 2+ ion close to the peroxide ion. Electrons in the peroxide ion will be strongly attracted towards the positive ion. This is then well on the way to forming a simple oxide ion if the right-hand oxygen atom (as drawn below) breaks off. We say that the positive ion polarises the negative ion. This works best if the positive ion is small and highly charged – if it has a high charge density.
