Since smaller cations` (Li^(+) , Na^(+))` stabilize smaller anions `(O^(2-))` and bigger cations `(K^(+) , Rb^(+)` and `Cs^(+)`) stabilize bigger anions `(O_(2)^(-))` , therefore, Li and Na form only oxides when heated in air but K , Rb and Cs forms superoxides .