Nearly every chemical reaction proceeds by a mechanism consisting
of several steps.
An elementary step is a chemical process that cannot be broken down
into simpler steps.
The molecularity of an elementary step is the number of atoms,
molecules, ions, or radicals involved in the step.
For elementary processes, the overall order of the rate law equals the
molecularity.
The temperature dependence of gas-phase reaction rates can be
understood through collision theory.
An approximate rate law for a given mechanism can often be deduced by
use of the rate-limiting step approximation or the steady-state
approximation.
Chain reactions have mechanisms that involve reactive intermediates
called chain carriers, which are produced as well as being consumed in
steps of the mechanism.
A photochemical chain reaction is initiated by the absorption of a photon.
