Alkali metals are highly reactive and their chemical reactivity increases down the group. Alkali metals burn vigorously in oxygen forming oxides. Lithium forms monoxide, sodium forms peroxide, and potassium form superoxide. The reactions are as follows:

4LiO +O₂→ 2Li₂O (oxide)

2Na + O₂→ Na₂O₂ (peroxide) with Na₂O as a minor product

K + O₂→ KO₂ (superoxide)

The formation of oxide to superoxide is

O₂^- + 1/2O₂→ O₂^2- + O₂→ 2O₂^-

Oxide peroxide superoxide

The increasing stability of the peroxide or superoxide, as the size of the metal ion increases, is due to the stabilisation of large anions by larger cations through lattice energy effects.