Well, recall from general chemistry, an elementary reaction is a reaction that takes place in a single step. And the important thing about elementary reactions that you have to remember is in any elementary reaction we can use the coefficient of that reactant to basically determine what the order of that reactant is within that rate law. So we can use the coefficient in front of the reactant to basically tell us what the exponent value is of that particular reactant in that rate law. So the rate law of the above elementary chemical reaction is usually described by using the rate law and the rate will depends on the rate constant k and usually depends on the concentration of the reactants. So in this lecture we're only going to look at elementary reactions. But you have to remember that if we're dealing with a multistep complicated reaction, then we cannot simply use the coefficients.