Alkali metals form different types of oxides with O2: Li → Li2O (oxide, O^2-), Na → Na2O2 (peroxide, O2^2-, bond order 1), K/Rb/Cs → MO2 (superoxide, O2^-, bond order 1.5, paramagnetic). Reason: larger cations stabilise larger anions (O^2- is smallest, O2^2- intermediate, O2^- largest). The lattice energy of the crystal must be favourable — large cation + large anion gives stable lattice. Na2O2 is a yellowish powder; used as bleach and in submarines for O2: 2Na2O2 + 2H2O → 4NaOH + O2. KO2 is orange; used in space/submarines: 4KO2 + 2CO2 → 2K2CO3 + 3O2 (dual function: removes CO2 and releases O2). Superoxides are paramagnetic (O2^- has unpaired electron).