The reactivity of alkali metals towards oxygen increases down the group as the atomic size increases . Thus , Li forms only lithium oxide `(Li_(2)O)` , sodium forms mainly sodium peroxide `(Na_(2)O_(2))` along with a small amount of sodium oxide while potassium forms only potassium superoxide `(KO_(2))`
`4 Li + O_(2) overset(Delta) (to) 2 Li_(2) O , 2 Na + O_(2) overset(Delta)(to) underset("(Major)") (Na_(2)O_(2)) + underset("(Minor)") (Na_(2)O)`
`K + O_(2) overset(Delta)(to) KO_(2)`
This is because of the following two reasons :
(i) As the size of the metal cations increases , the positive field around it becomes weaker and weaker thereby permitting the intially formed oxide `(O^(2-))` ion to combine with another oxygen atom to form first peroxide ion `(O_(2)^(-))` and then superoxide `(O_(2)^(-))` ion .
`underset("Oxide") (O^(2-)) overset(1//2 O_(2)) (to) underset("Peroxide")(to) (O_(2)^(2-)) overset(O_(2))(to) underset("Superoxide")(2O_(2)^(-))`
(ii) Since larger cations stabilize larger anions due to higher lattice energies , therefore , the stability increases from oxide `to` peroxide`to` superoxide as the size of the metal cation increases down the group and the size of the anion increases from oxide `to` peroxide `to` superoxide .
