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

Explanation

The stability of peroxide or superoxide increases as the size of metal ion increases i.e., `KO_(2) lt RbO_(2) lt CsO_(2)` ltbr. The reacivity of alkali metals towards oxygen to form different oxides is due to strong positive field around each alkali metal cation. `Li^(+)` is the smallest. It does not allow `O^(2-)` ion to react with `O_(2)` further. `Na^(+)` is larger than `Li`, its positive field is weaker than `Li^(+)`. It cannot prevent then conversion of `O^(2-)` into `O_(2)^(2-)`. The largest `K^(+),Rb^(+)` and `Cs^(+)` ions permit `O_(2)^(2-)` ion to react with `O_(2)` formign superoxide ion `O_(2)^(-)`.
`underset("Oxide")(O_(2)^(2-))overset((1)/(2)O_(2))rarrunderset("Peroxide")(O^(2-))overset(O_(2))rarr underset("Superoxide")(2O_(2)^(-))`
Futhermore, increased stability of the peroxide or superoxide with increase in the size of metal ion is due to the stabilisation of large anion by larger cations through lattice energy effect.