The molecularity of the following elementary reaction is

\(NH_4NO_2 → N_2 + 2H_2O\)

one

The correct answer is option 2. one.

In chemistry, molecularity refers to the number of molecules or ions that participate as reactants in an elementary reaction, which is a single-step reaction that occurs on a molecular scale. Molecularity is a concept used primarily in the context of reaction mechanisms, particularly in elementary reactions.

In the given reaction:

\(\text{NH}_4\text{NO}_2 \rightarrow \text{N}_2 + 2\text{H}_2\text{O}\)

This is a decomposition reaction where one molecule of ammonium nitrite (\( \text{NH}_4\text{NO}_2 \)) breaks down into nitrogen gas (\( \text{N}_2 \)) and two molecules of water (\( \text{H}_2\text{O} \)).

The reaction involves the decomposition of a single molecule of ammonium nitrite into the products. There are no other reactants involved, and the entire reaction occurs in a single step. Therefore, the molecularity of this reaction is one.

Understanding the molecularity of a reaction helps chemists characterize the complexity and mechanism of chemical reactions. In this case, the reaction is unimolecular, meaning it involves the decomposition of a single molecule in a single step. This information is important for studying and predicting the behavior of chemical systems.