Question

A metal on combustion in air vigorously forms a superoxide X . Name the metal.

• A. Li
• B. Na
• C. Rb
• D. None of the above

Answer 1

The correct option is C Rb

The lithium metal in the air form Lithium monoxide.
The equation for the formation of the peroxide is:
$4Li+O_2⟶2L{i}_{2}O$
The sodium metal in the excess air form sodium peroxide.
The equation for the formation of the peroxide is:
$2Na+O_2⟶N{a}_2{O}_2$

The Rubidium metal in the air form rubidium superoxide.
The equation for the formation of the peroxide is:
$Rb+O_2⟶Rb{O}_2$
Hence, correct options is c.

Theory
Reactivity towards air:
Alkali metals tarnish in dry air due to the formation of their oxides which in turn react with moisture
to form hydroxides.
They burn vigorously in oxygen forming oxides:
Lithium monoxide
$4Li+O_2⟶2L{i}_{2}O$
Sodium peroxide
$2Na+O_2⟶N{a}_2{O}_2$
Other superoxide:
$M+O_2⟶M{O}_2$
Where M = K, Rb, Cs
The superoxide $O_2$ ion is stable only in the presence of larger cations such as K, Rb, Cs.