Question

The stability of peroxide and superoxide of alkali metals increases as we go down the group. Explain giving a reason.

Answer 1

As the size of alkali metal ions increases the stability of peroxides and superoxide increases i.e. stability order for superoxide \( KO_{2}<RbO_{2}<CsO_{2} \).
This is due to the stabilization of larger anions by larger cations.

The reactivity of alkali metals towards oxygen to form different oxides is due to strong positive field around each alkali metal cation.

As we move from \( Li^{+}\;to\;Cs^{+}\) the size of the cations increases. \( Li^{+}\) is the smallest cation it does not allow \( O^{2-} \) ion to react with \(O_{2}\) further.
Hence, it forms oxide.

\(Na^{+}\) is larger than \( Li^{+}\), but its positive field is weaker than \( Li^{+}\), so it cannot prevent the conversion of \( O^{2-} ~into~ O_{2}^{2-} \) (peroxides).


The largest \( K^{+},Rb^{+},\;and\;Cs^{+}~ions ~permit~O_{2}^{2-} \) ion to react with \(O_{2}\) forming superoxide ion \(O_{2}^{-}\).

Furthermore, increased stability of the peroxide or superoxide with increase in the size of the metal ion is due to the stabilisation of larger anion by larger cation (high lattice energy).