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Alkali metals form different oxides when burned in excess O2, governed by cation size:

• Li → Li2O (oxide, $O^{2-}$) — $Li^+$ too small for larger anions
• Na → Na2O2 (peroxide, $O2^{2-}$, bond order 1, diamagnetic)
• K, Rb, Cs → MO2 (superoxide, $O2^-$, bond order 1.5, paramagnetic)

The principle: larger cations stabilise larger anions through favourable lattice energy. With limited O2, all form M2O first.

KO2 in life support: 4KO2 + 2CO2 → 2K2CO3 + 3O2. Dual function — absorbs exhaled CO2 and releases O2 for breathing. Used in submarines, spacecraft, and emergency breathing equipment.

Na2O2 for O2: 2Na2O2 + 2H2O → 4NaOH + O2. Also reacts with CO2: 2Na2O2 + 2CO2 → 2Na2CO3 + O2. Less preferred than KO2 because it doesn't match CO2 consumption with O2 production as efficiently.

Superoxide $O2^-$ is paramagnetic (1 unpaired electron). Peroxide $O2^{2-}$ is diamagnetic (all electrons paired). This is a common JEE question.