Correct Answer - Option 3 : Rb
Concept:
Metal X is rubidium (Rb). In excess of air, it forms RbO2 (X). X is a superoxide that have \({\rm{O}}_2^ -\) ion. It is due to the stabilization of large anion by large cations through lattice energy effects. RbO2 (X) is easily hydrolyzed by water to form the hydroxide, H2 O2 and O2.
The reaction involved are as follows:
Rb + O2 → RbO2 (superoxide)
(X)
2RbO2 + 2H2 O → 2RbOH + H2O2 + O2
(X)
