Oxides and Hydroxides

When burnt in excess air, each alkali metal favours a different class of oxide, though minor amounts of the others can also form: lithium gives predominantly the normal oxide $Li_2O$ (with a little peroxide $Li_2O_2$), sodium gives mainly the peroxide $Na_2O_2$ (with some superoxide $NaO_2$), while potassium, rubidium and caesium give predominantly the superoxide $MO_2$. Under carefully controlled conditions, the pure normal oxide ($M_2O$), peroxide ($M_2O_2$) or superoxide ($MO_2$) can be isolated for any of them. The trend — a growing preference for the larger, more diffuse peroxide and superoxide ions as the metal ion itself grows larger — is explained by lattice energy: a large anion is stabilised most effectively by a similarly large cation.

All three classes of oxide hydrolyse readily in water to give the hydroxide, with peroxides and superoxides also liberating hydrogen peroxide (and, for superoxides, oxygen as well):

Pure normal oxides and peroxides are colourless, but the superoxides are yellow or orange and, unusually for these compounds, paramagnetic. Sodium peroxide finds wide use as an oxidising agent in inorganic chemistry. …