Science, Tech, Math › Science Elementary Reaction Definition Understanding Elementary Reactions Share Flipboard Email Print Radioactive decay is a simple example of an elementary reaction. Dorling Kindersley/ Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated December 05, 2019 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 Sources 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").