Compare with net chemical reaction. A reaction that occurs in a single step. Equations for elementary reactions show the actual molecules, atoms, and ions that react on a molecular level.
The IUPAC Compendium of Chemical Terminology defines elementary reactionhttp://www.iupac.org/goldbook/E02035.pdf IUPAC Gold Book definition of elementary reaction as "a reaction for which no reaction intermediates have been detected or need to be postulated in order to describe the chemical reaction on a molecular scale. An elementary reaction is assumed to occur in a single step and to pass through a single transition state".