Question

Define the Elementary Reaction.

Answer 1

It is a single step reaction. It is a reaction which has a single transition state. In this reaction, No reaction intermediates are required or formed in the reaction.

Complete step by step answer:
As we know in a chemical reaction, reactants react in presence or absence of catalyst to form the required products.
In the same way, an elementary reaction is a type of chemical reaction in which one or more chemical species react directly to form products in a single step with a single transition state, which means reactants react directly with each other to give product without forming any intermediate compound.
No reaction intermediates are detected or intermediates of variable lifetime are formed which are not required to describe the complete reaction.
We can see example of such reactions below:
${C_2}{H_2}OH + C{H_3}COOH \to C{H_3}COO{C_2}{H_5} + {H_2}{O_{}}$
Here we can see two reactants combined directly to form products. Basically ${H^ + }$ from $C{H_3}COOH$ and $O{H^ - }$ from ${C_2}{H_2}OH$ combined to form ${H_2}O$ and rest other compounds combined themselves to form second product.
-Cis-trans Isomerization
-Thermal decomposition
-Nucleophilic substitution

Note:
For more clear view we can see the types of elementary reaction, which are:
Unimolecular Reaction: In this reaction, the molecule rearranges itself or dissociates thus forming one or more products. It is a first order reaction. Radioactivity decay which involves emission of particles from one atom is one of its examples.
Bimolecular Reaction: In this reaction, two particles react to form one or more products. It is also known as second order reaction. These reactions are common in organic chemistry. Nucleophilic substitution is an example of such a reaction.
Termolecular Reaction: This reaction involves three particles which collide and react to form products. It is a third order reaction.
Example is: $2NO + {O_2} \to 2N{O_2}$