The main oxides formed on combustion of $\mathrm{Li}, \mathrm{Na}$ and K in excess of air are, respectively:

(C) $\mathrm{Li}_{2} \mathrm{O}, \mathrm{Na}_{2} \mathrm{O}_{2}$ and $\mathrm{KO}_{2}$

(A) $\mathrm{LiO}_{2}, \mathrm{Na}_{2} \mathrm{O}_{2}$ and $\mathrm{K}_{2} \mathrm{O}$

(B) $\mathrm{Li}_{2} \mathrm{O}_{2}, \mathrm{Na}_{2} \mathrm{O}_{2}$ and $\mathrm{KO}_{2}$

(D) $\mathrm{Li}_{2} \mathrm{O}, \mathrm{Na}_{2} \mathrm{O}$ and $\mathrm{KO}_{2}$

JEE Main · 2016 · 03 Apr 2016 · Offline · s-Block Elements

The main oxides formed on combustion of

Li, Na

and K in excess of air are, respectively:

Answer: (C)

Burning in excess air, each alkali metal forms the oxide its cation can best stabilise. The small lithium ion holds only the simple oxide

Li X_{2} O

; the larger sodium ion stabilises the peroxide

Na X_{2} O X_{2}

; and the still larger potassium ion stabilises the superoxide

KO X_{2}

, since bigger cations support the bigger

O X_{2} X^{2 -}

and

O X_{2} X^{-}

anions. The products are therefore

Li X_{2} O

Na X_{2} O X_{2}

and

KO X_{2}

JEE Main · 2016 · 03 Apr 2016 · Offline · Single correct option