When heated in air, the alkali metals form various oxides. Mention the oxides formed by Li, Na and K.
The reactivity of alkali metals towards oxygen increases on moving down the group with the increase in atomic size. Thus , Li forms only lithium oxide Li2O but the sodium forms mainly sodium peroxide Na2O2 along with a small amount of sodium oxide while potassium forms only potassium superoxide (KO2)
4Li + O2△−→ 2Li2O6Na + 2 O2△−→ Na2O2Sodium peroxiode (major) + 2 Na2O Monoxide(minor)K+O2△−→KO2Potessiumsuper oxide+K2O2Peroxide+K2(O)Monoxide
The superoxede,O−2 ion is stable only in presence of large cations such as K, Rb etc.