Elementary reaction

An elementary reaction occurs in a single molecular event, without proceeding through separately distinguishable reaction steps. Its molecularity determines how many reactant particles participate in that event.

Molecularity: The number of reactant particles that participate in a single elementary reaction event. It classifies elementary reactions as unimolecular, bimolecular, or termolecular.
Chemical kinetics: The study of reaction rates and how they depend on conditions and reaction pathways. Elementary reactions are the individual events whose rates kinetics seeks to explain.
Overall reaction: The net chemical change obtained by combining the steps of a reaction mechanism. An overall reaction summarizes net change but does not establish that it occurs in one event.
Lindemann mechanism: A pressure-dependent mechanism that models how unimolecular gas reactions are activated by collisions. It shows how an apparently single-molecule reaction can arise through multiple elementary steps.
Rate law: An equation relating a reaction rate to reactant concentrations and, sometimes, other variables. For an elementary step, its molecularity determines the concentration exponents in its rate law.
Chemical reaction mechanism: A sequence of molecular steps that describes how reactants are converted into products. A mechanism is assembled from elementary reactions and any intermediates they produce.
Unimolecular reaction: An elementary reaction in which one reactant molecule undergoes a molecular change. It is the one-particle case of an elementary reaction.
Haber process: An industrial process that synthesizes ammonia from nitrogen and hydrogen over an iron-based catalyst. Its catalytic mechanism is analyzed as a sequence of elementary surface reactions.
Transition state: A high-energy molecular configuration at the boundary between reactants and products along a reaction pathway. A single elementary event passes through a transition state rather than a separately stable intermediate.
Chemical species: A chemically distinct type of atom, molecule, ion, or molecular fragment. Reactant and product species identify the particles participating in an elementary event.

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