Oxide Types of Alkali Metals

When alkali metals burn in excess oxygen, they form different types of oxides depending on cation size:

$Li + O_2 \rightarrow Li_2O\ \ (normal\ oxide,\ $O^{2-}$)$ $Na + O_2 \rightarrow Na_2O_2\ \ (peroxide,\ O_2^{2-})$ $K + O_2 \rightarrow KO_2\ \ (superoxide,\ O_2^{-})$ $Rb + O_2 \rightarrow RbO_2\ \ (superoxide)$ $Cs + O_2 \rightarrow CsO_2\ \ (superoxide)$

Types of Oxygen Anions

Ion	Name	Charge	O oxidation state	Magnetic	Bond Order
$O^{2-}$	Oxide	-2	-2	Diamagnetic	—
$O_{2}^{2-}$	Peroxide	-2	-1	Diamagnetic	1
$O_{2}^{-}$	Superoxide	-1	-1/2	Paramagnetic	1.5

Why different oxides? Determined by lattice energy optimization:

Small $Li^{+}$ → best paired with small $O^{2-}$ (oxide) for maximum lattice energy
Intermediate $Na^{+}$ → stabilizes $O_{2}^{2-}$ (peroxide) best
Large $K^{+}$ / $Rb^{+}$ / $Cs^{+}$ → stabilizes large $O_{2}^{-}$ (superoxide) best

Reactions with water and $CO_{2}$ : $2Na_2O_2 + 2H_2O \rightarrow 4NaOH + O_2\uparrow$ $2Na_2O_2 + 2CO_2 \rightarrow 2Na_2CO_3 + O_2\uparrow$ $4KO_2 + 2H_2O \rightarrow 4KOH + 3O_2\uparrow$ $4KO_2 + 2CO_2 \rightarrow 2K_2CO_3 + 3O_2\uparrow$

Mnemonic: "LON-PS-S" — Li = Oxide (normal), Na = Peroxide, K/Rb/Cs = Superoxide