Question

Which of the following exists as a monomer molecule only?
A.Nitrogen \[\left( {{\text{III}}} \right)\] oxide
B.Phosphorus \[\left( {\text{V}} \right)\] oxide
C.Arsenic \[\left( {{\text{III}}} \right)\] oxide
D.Antimony \[\left( {\text{V}} \right)\] oxide

Answer 1

Hint:The oxide that exists in only its monomer is the oxide made by the smallest member of this family. The compounds with the oxidation of the central atom as 5 usually exist as dimer.

Complete step by step solution:
The molecular formula for the compound Nitrogen \[\left( {{\text{III}}} \right)\] oxide is \[{{\text{N}}_2}{{\text{O}}_3}\] . The oxidation state of nitrogen in this compound is 3. It is formed by mixing nitrogen monoxide and nitrogen dioxide. It is stable in this form and hence does not polymerise to form \[{{\text{N}}_4}{{\text{O}}_6}\] . So the molecule that exists as the monomer unit only is Nitrogen \[\left( {{\text{III}}} \right)\] oxide.
Let us look at other options as well:
The molecular formula for the compound Phosphorus \[\left( {\text{V}} \right)\] oxide is \[{{\text{P}}_2}{{\text{O}}_5}\] . The oxidation state of Phosphorus in this compound is 5. It exists in its dimer state as \[{{\text{P}}_4}{{\text{O}}_{10}}\] . It exists in more polymeric forms as well.
The molecular formula for the compound Arsenic \[\left( {{\text{III}}} \right)\] oxide is \[{\text{A}}{{\text{s}}_2}{{\text{O}}_3}\] . The oxidation state of Arsenic in this compound is 3. It exists in its dimer state as \[{\text{A}}{{\text{s}}_4}{{\text{O}}_6}\] . It has the same structure as \[{{\text{N}}_2}{{\text{O}}_3}\]. It also exists in 2 other polymeric forms.
The molecular formula for the compound Antimony \[\left( {\text{V}} \right)\] oxide is \[{\text{S}}{{\text{b}}_2}{{\text{O}}_5}\]. The oxidation state of Antimony in this compound is 5. It can also exist in the form of polymer as like other oxide of this group in plus 5 oxidation state.

Hence, the correct option is A.

Note:
Nitrogen \[\left( {{\text{III}}} \right)\] oxide is a deep blue coloured solid. It is also known as dinitrogen trioxide. It is more stable at the lower temperature, at higher temperature the constituent gases that make dinitrogen trioxide is more stable.