By Sunil Bhardwaj

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The number of reacting species (atoms, molecules or ions) which collide simultaneously to bring about a chemical reaction is known as molecularity of a reaction.

$$ { H }_{ 2 }{ O }_{ 2 }(l) \longrightarrow { H }_{ 2 }{ O }(l) + \frac12 { O }_{ 2 } (g) \qquad \text{ (Unimolecular reaction) } $$ $$2HI(g) \longrightarrow { H }_{ 2 }(g) + { I }_{ 2 } (g) \qquad \text{ (Bimolecular reaction) } $$ $$ 2NO(g) + { O }_{ 2 }(g) \longrightarrow 2{ NO }_{ 2 }(g) \qquad \text{ (Trimolecular reaction) } $$

When the reaction involves decomposition of a single species, it will be unimolecular reaction.

Similarly, when the reaction involves collision between two species, it will be a bimolecular reaction and if three species take part in the collision, it is called trimolecular reaction.

Reactions involving three or more molecules are uncommon. This is because occurrence of such reactions require simultaneous collisions of three or more molecules. The chances of occurrence of such collisions are very small. Reactions involving three or more molecules should be slow.