Molecularity is the total number of reactant species taking part in an elementary chemical reaction that must collide simultaneously to bring about the reaction.

• It is a theoretical property of an elementary step.
• It must be a positive integer ($1,2$ or $3$). It cannot be zero, fractional, or negative.
• Molecularity $>3$ is extremely rare because the probability of more than three molecules colliding simultaneously with proper orientation is negligible.