The molecularity of an elementary step describes the number of reactive partners in the elementary step. For example, the above elementary step is called bimolecular because two molecules collide. Commonly, elementary steps are mono-, bi-, or termolecular. The probability of four molecules colliding at exactly the same place and time is so small that we can safely assume that no reaction will ever be tetramolecular. Because take up a large amount of space, we will represent elementary steps in this SparkNote as normal reactions with molecular formula line equations. You will know from the context (i.e. talking about the steps of a mechanism) whether the reaction is an elementary step or an overall reaction.