The stability of peroxide and superoxide of alkali metals increase as we go down to group. Explain giving reason.

The stability of peroxide or superoxide increases as the size of metal ion increases i.e.,
${\mathrm{KO}}_{2}<{\mathrm{RbO}}_{2}<{\mathrm{CsO}}_{2}$
The reactivity of alkali metals toward oxygen to form different oxides is due to strong positive field around each alkali metal cation, Li+ is the smallest, it does not allow O2- ion to react with O2 further. Na+ is larger than Li, its positive field is weaker than Li+. It cannot prevent the coversion of O2- into ${{\mathrm{O}}_{2}}^{2-}$.
The largest K+, Rb+ and Cs+ ions permit ${\mathrm{O}}_{2}^{2-}$ ion to react with O+ forming superoxide ion ${\mathrm{O}}_{2}^{-}.$
$\underset{\mathrm{Oxide}}{{\mathrm{O}}_{2}^{2-}}\stackrel{\frac{1}{2}{\mathrm{O}}_{2}}{\to }\underset{\mathrm{Peroxide}}{{\mathrm{O}}^{2-}}\stackrel{{\mathrm{O}}_{2}}{\to }\underset{\mathrm{Superoxide}}{2{\mathrm{O}}_{2}^{-}}$
Furthermore, increased stabiliity of the peroxide or superoxide with increase in the size of metal ion is due to the stabilisation of large anions by larger cations through lattice energy effect.