Elementary Reaction Definition

Understanding Elementary Reactions

Radioactive decay scheme
Radioactive decay is a simple example of an elementary reaction.

 Dorling Kindersley/ Getty Images

Elementary Reaction Definition

An elementary reaction is a chemical reaction where reactants form products in a single step with a single transition state. Elementary reactions may combine to form complex or nonelementary reactions.

Key Takeaways: What Is an Elementary Reaction?

  • An elementary reaction is a type of chemical reaction in which the reactants directly form the products. In contrast, a nonelementary or complex reaction is one in which intermediates form, which go on to form the final products.
  • Examples of elementary reactions include cis-trans isomerization, thermal decomposition, and nucleophilic substitution.

Elementary Reaction Examples

Types of elementary reactions include:

Unimolecular Reaction - a molecule rearranges itself, forming one or more products

A → products

examples: radioactive decay, cis-trans isomerization, racemization, ring opening, thermal decomposition

Bimolecular Reaction - two particles collide to form one or more products. Bimolecular reactions are second-order reactions, where the rate of the chemical reaction depends on the concentration of the two chemical species that are the reactants. This type of reaction is common in organic chemistry.

A + A → products

A + B → products

examples: nucleophilic substitution

Termolecular Reaction - three particles collide at once and react with each other. Termolecular reactions are uncommon because it's unlikely three reactants will simultaneously collide, under the right condition, to result in a chemical reaction. This type of reaction is characterized by:

A + A + A → products

A + A + B → products

A + B + C → products


  • Gillespie, D.T. (2009). A diffusional bimolecular propensity function. The Journal of Chemical Physics 131, 164109.
  • IUPAC. (1997). Compendium of Chemical Terminology, 2nd ed. (the "Gold Book").
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Helmenstine, Anne Marie, Ph.D. "Elementary Reaction Definition." ThoughtCo, Aug. 28, 2020, thoughtco.com/definition-of-elementary-reaction-605078. Helmenstine, Anne Marie, Ph.D. (2020, August 28). Elementary Reaction Definition. Retrieved from https://www.thoughtco.com/definition-of-elementary-reaction-605078 Helmenstine, Anne Marie, Ph.D. "Elementary Reaction Definition." ThoughtCo. https://www.thoughtco.com/definition-of-elementary-reaction-605078 (accessed January 28, 2023).