When heated in air, the alkali metals form various oxides. Mention the oxides formed by Li, Na and K.
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→ O22- + 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.