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Unimolecular and bimolecular steps describe different types of elementary reaction rates based on the number of reactant molecules involved in the process. A unimolecular step is an elementary reaction in which a single reactant molecule is involved. It undergoes a change to produce one or more products. The rate of a unimolecular reaction depends solely on the concentration of that single reactant. An example of a unimolecular reaction is the isomerization of cyclopropane to propene. A bimolecular step is an elementary reaction in which two reactant molecules are involved. These two molecules collide with each other and, providing they have sufficient energy and orientation, they undergo a change to produce one or more products. The rate of a bimolecular reaction depends on the concentration of each of the two reacting species. An example of a bimolecular reaction is the reaction between hydrogen and iodine molecules to produce hydrogen iodide.

Termolecular steps are elementary reactions that involve three reactant molecules colliding with one another and directly participating in the reaction to form products. These reactions are infrequent in comparison to unimolecular and bimolecular steps because the probability of three molecules colliding simultaneously with the correct orientation and sufficient energy is very low. Moreover, termolecular steps generally have higher activation energy barriers than their unimolecular and bimolecular counterparts. The combined effect of lower probability of three-body collisions and increased activation energy requirements result in a significantly lower reaction rate, making termolecular steps very rare in chemical reactions.