Reaction Mechanisms
An elementary process is a one-step process in which the product particles are in most cases the direct result of collisions of only 2 reactant particles. There are three particles collisions but they are rare. The chances of 4 or more particles forming a one-step reaction reach infinity. The reaction mechanism for
 
H2(g) + Cl2(g) ---> 2 HCl(g) is made up of 4 elementary steps.