Question

A nitrate of an alkali metal $M$ on heating gives ${{O}_{2}}$,$N{{O}_{2}}$ and ${{M}_{2}}O$. The metal $M$ will be:
a.) $Na$
b.) $K$
c.) $Rb$
d.) $Li$

Answer 1

Hint: The question is based on chemical properties of group $1$ elements which are known as alkali metals and their similarities and relationship with group $2$ elements of the periodic table.
- Some elements of group $1$ shows similar properties to group $2$ and the relationship is known as diagonal relationship.
- Group $1$ elements or alkali metal nitrates decompose to give nitrite metal salt and oxygen.


Complete Solution :
- Alkali metal nitrates differ in their way of decomposition than group $2$ nitrates but due to the diagonal relationship between both elements of group $1$ differ in their way of decomposition.
- Lithium is a group $1$ compound whose nitrate decomposes differently showing resemblance to magnesium of ${{2}^{nd}}$ group.
- Group 1 compounds are more stable to heat than the corresponding compounds in Group $2$. You will often find that the lithium compounds behave similarly to Group $2$ compounds, but the rest of Group $1$ are in some way different.
- Group $1$ compound nitrate decomposes to give nitrite metal salt and oxygen.
$2KN{{O}_{3}}\to 2KN{{O}_{2}}+{{O}_{2}}$

Whereas lithium nitrates decompose to give metal oxide, $N{{O}_{2}}$ and oxygen gas the same as the decomposition of group 2 nitrates.
$4LiN{{O}_{3}}_{\left( S \right)}\to 2L{{i}_{2}}{{O}_{\left( S \right)}}+4N{{O}_{2}}_{\left( g \right)}+2{{O}_{2}}_{\left( g \right)}$
So, the correct answer is “Option D”.

Note: As you go down the Group, the decomposition gets more difficult, and you have to use higher temperatures.
- Decomposition of group two nitrates takes place with evolution of brown fumes of $N{{O}_{2}}$.
- Because of its relatively small size, the lithium cation is very polarizing, which favors the formation of the oxide.