Molecularity refers to the number of moles of reactant molecules involved in an elementary reaction step. For instance, in the reaction where 1 mole of carbon dioxide gas reacts with 1 mole of water to produce 1 mole of carbonic acid, we have a total of 2 moles of reactants. This leads us to classify the molecularity of this reaction as bimolecular, since it involves two reactant molecules.

When determining molecularity, specific prefixes are used to denote the number of reactants. A single reactant corresponds to unimolecular, while two reactants are termed bimolecular. It's important to note that while the prefix "di-" is commonly used in naming covalent compounds (as in carbon dioxide), for molecularity, we use "bi" to indicate two moles of reactants.

If a reaction involves three moles of reactants, it is classified as termolecular. Although the prefix "tri-" is typically used for three in other contexts, the term "trimolecular" does not exist; instead, we simply refer to it as termolecular. Thus, to determine the molecularity of any elementary step, one must count the total number of reactive molecules involved.