Question

Oxide, peroxide and superoxide- all the three can be formed by:
(A) $Li,K$
(B) $Na,K$
(C) $K,Rb$
(D) $Na,Cs$

Answer 1

When metal (having valency $+ 1$ ) forms a molecule with oxygen then the oxygen can show more than one type of valency i.e. $- 2, - 1, - \dfrac{1}{2}$ . Based on these valencies the molecule formed is known as oxides, peroxides and superoxides.

Complete step by step solution:
Let us first talk about oxides, peroxide and superoxides.
Oxide: The molecule formed by metal (having valency $+ 1$ ) and a single atom oxygen and in which the valency of oxygen is $- 2$ .
Peroxide: The molecule formed by metal (having valency $+ 1$ ) and two atoms of oxygen and in which the valency of each atom of oxygen is $- 1$ . And both the oxygen atoms are bonded together.
Superoxide: The molecule formed by metal (having valency $+ 1$ ) and two atoms of oxygen and in which the valency of each oxygen atom is $- \dfrac{1}{2}$ . And both the oxygen atoms are bonded together.
Generally the metals having valency $+ 1$ are known as alkali metals and they are first group elements. The elements present in this group are lithium, sodium, potassium, rubidium, caesium and francium.
Now, the elements lithium and sodium form oxide and peroxide only while potassium, rubidium and caesium form superoxides but potassium, rubidium can form oxides and peroxides as well. Hence, oxide, peroxide and superoxide- all the three can be formed by potassium ( $K$ ) and rubidium ( $Rb$ ).
Hence option C is correct.

Note:
The stability order of oxides, peroxides and superoxides are as follows:
Oxides $>$ peroxides $>$ superoxides i.e. oxides are more stable than its peroxides and superoxides form.