When heated in excess of air, alkali metals form different types of oxides depending upon the nature of the metal. Which of the following alkali metals form surperoxides ?

Options

A. All alkali metals except Li
B. All alkali metals except Li and Na
C. All alkali metals except Li, Na and K
D. All alkali metals except Fr

Answer

B

Explanation

The reactivity of alkali metals with oxygen increases on moving down the group. Li metal forms the monoxide (and some peroxide).
`4Li(s)+O_(2)(g) to 2Li_(2)O(s)`
Na metal forms peroxide (and some monoxide)
`2Na+O_(2)(g) overset(300^(@)C)to Na_(2)O_(2)`
The other alkali metals (K, Rb, and Cs) form superoxides of the type `MO_(2)`
`M(s)+O_(2)(g) to MO_(2)`
(m=K, Rb and Cs)
On account of very small size, `Li^(+)` ion carries a strong positive field around it which affects the negative charge of `O^(2-)` so strongly that it does not permit the oxide anion `O_(2)^(-)` to combine with another atom to form peroxide ion `O_(2)^(2-)` :
`underset("Oxide")O^(2-)overset((1)/(2)O_(2))to underset("Peroxide")O_(2)^(2-)`
In other words, `Li^(+)` ion (by virtue of its small size) is not able to surround itself by sufficient `O_(2)^(2-)` (peroxide ions) to give a stable crystal lattice and , consequently, only the monoxide exists.
`Na^(+)` ion has relatively weaker positive field around it (because of relative larger size than `Li^(+)` ion ) which allows `O^(2-)` to combine with another O atom to form the peroxide ion `(O_(2)^(2-))`. The still larger `K^(+), Rb^(+), " and " Cs^(+)` ions with still weaker positive fields around them allow peroxide ions `(O_(2)^(2-))` to combine with another O atom to form superoxide `O_(2)^(-)` :
`underset("Oxide")O^(2-)overset((1)/(2)O_(2))to underset("Peroxide")O_(2)^(2-) overset (O_(2))to 2O_(2)^(-)`
In other words, the large ions of K, Rb, and Cs are able to form stable structures with the superoxide ion -the largest of the three anions.
Thus, as the size of the metal ion increases, the stability of peroxides and superoxides increases due to the stabilization of larger anions by larger cations through higher lattice enthalpies.