text: For an elementary step, there is a relationship between stoichiometry and rate law, as determined by the law of mass action.  Almost all elementary steps are either unimolecular or bimolecular.  For a unimolecular step
  the reaction rate is described by , where  is a unimolecular rate constant. Since a reaction requires a change in molecular geometry, unimolecular rate constants cannot be larger than the frequency of a molecular vibration.  Thus, in general, a unimolecular rate constant has an upper limit of k ≤ ~10 s.  
  For a bimolecular step
  the reaction rate is described by , where  is a bimolecular rate constant. Bimolecular rate constants have an upper limit that is determined by how frequently molecules can collide, and the fastest such processes are limited by diffusion.  Thus, in general, a bimolecular rate constant has an upper limit of k ≤ ~10 Ms.  
  For a termolecular step
  the reaction rate is described by , where  is a termolecular rate constant.
  There are few examples of elementary steps that are termolecular or higher order, due to the low probability of three or more molecules colliding in their reactive conformations and in the right orientation relative to each other to reach a particular transition state.  There are, however, some termolecular examples in the gas phase. Most involve the recombination of two atoms or small radicals or molecules in the presence of an inert third body which carries off excess energy, such as O +  +  →  + .  One well-established example is the termolecular step 2 I +  → 2 HI in the hydrogen-iodine reaction. In cases where a termolecular step might plausibly be proposed, one of the reactants is generally present in high concentration (e.g., as a solvent or diluent gas).