Elementary reactions and composite reactions are two fundamental types of chemical reactions, distinguished primarily by the number of steps involved in their processes.

Elementary reactions represent the most straightforward category of chemical reactions. They occur in a single, direct step without the formation of any intermediate products. In these reactions, reactants are converted directly into products. The rate of an elementary reaction is dependent on the concentrations of the reactants. For instance, the reaction between hydrogen ($H_2$) and iodine ($I_2$) to form hydrogen iodide ($HI$) is an elementary reaction. This process occurs in a single step, where $H_2$ and $I_2$ molecules collide to produce $HI$.

In contrast, composite reactions—often referred to as complex reactions—are characterized by multiple sequential steps. Each of these steps is itself an elementary reaction, and the overall reaction can be viewed as the sum of these individual steps. Composite reactions frequently involve the formation of intermediate products, which are generated in one step and subsequently consumed in another. The rate of a composite reaction is dictated by the slowest step, known as the rate-determining step. A pertinent example of a composite reaction is the reaction between nitrogen dioxide ($NO_2$) and carbon monoxide ($CO$) to produce nitrogen monoxide ($NO$) and carbon dioxide ($CO_2$). This reaction unfolds in two distinct steps: first, $NO_2$ molecules collide to form nitrogen tetroxide ($N_2O_4$) and oxygen ($O_2$); then, in the second step, $N_2O_4$ reacts with $CO$ to yield $NO$ and $CO_2$.

In summary, the primary distinction between elementary and composite reactions lies in the number of steps involved. Elementary reactions occur in a single step, while composite reactions consist of multiple steps. Grasping this difference is vital for accurately predicting the rate of a reaction and understanding its underlying mechanism.