Molecularity of Elementary Steps HL Only
Quick Notes
- Molecularity refers to the number of reactant particles involved in a single elementary step in a mechanism.
- Types of molecularity:
- Unimolecular: 1 particle.
- Bimolecular: 2 particles.
- Termolecular: 3 particles.
- Termolecular steps are rare due to the low probability of three particles colliding simultaneously with correct orientation and energy.
Full Notes
RECAP: Many chemical reactions don’t happen in one step, they occur through a sequence of elementary steps:

Each step involves the breaking or forming of a small number of bonds. The overall reaction is the sum of all elementary steps that occur.
How the steps link together is referred to as the ‘reaction mechanism’.
What Is Molecularity?
Molecularity describes how many reactant species are involved in an elementary step of a reaction mechanism.
It applies only to individual steps, not to the overall reaction.
Types of Molecularity
There are three main examples of molecularity - unimolecular, bimolecular and trimolecular.

Unimolecular:
Involves 1 particle.
Example: A → product or intermediate.
Common in decomposition reactions.

Unimolecular steps usually involve just one reacting species, but they can still be triggered by collisions with non-reactive particles, like solvent molecules. These collisions may supply energy to break bonds, but because the solvent doesn’t change chemically, it isn’t considered a reactant and its concentration doesn’t affect the rate.
Bimolecular:
Involves 2 particles.
Example: Example: A + B → product or intermediate.
Most common type in collisions and substitution reactions.
Termolecular:
Involves 3 particles colliding simultaneously.
Example: A + B + C → product or intermediate.
Note that termolecular processes are very rare, as it is statistically unlikely for three particles to collide with the proper orientation and energy at the same time.
Summary
- Molecularity is the number of reactant particles in a single elementary step.
- Types of molecularity:
- Unimolecular: 1 particle.
- Bimolecular: 2 particles.
- Termolecular: 3 particles.
- Termolecular reactions are rare due to low probability of three effective collisions.