Alkali metals on burning in air give monoxide, peroxide or superoxide
Li forms monoxide
$2Li +\frac{1}{2}O_{2} \to Li_{2}O$
Na form peroxide as well as monoxide
$2Na+O_{2} \to Na_{2}O_{2}$
$2Na+\frac{1}{2}O_{2} \to Na_{2}O$
$K$ , $Rb$ ,and $Cs$ form superodxide
$M(=K, Rb, Cs)+O_{2} \to MO_{2}$