Molecularity is the number of molecules that must come together and collide in a given elementary step, taking the value one, two, or, rarely, three. It applies only to a single elementary step, not to an overall reaction. For an elementary step the order equals the molecularity, but this is not generally true for an overall reaction.

分子数是在某一基元步骤中必须聚集并碰撞的分子数目,取值为一、二,或极少为三。它只适用于单个基元步骤,而不适用于总反应。对于基元步骤,级数等于分子数,但对总反应一般不成立。