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An elementary reaction is a
chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
in which one or more
chemical species A chemical species is a chemical substance or ensemble composed of chemically identical molecular entity, molecular entities that can explore the same set of molecular energy levels on a characteristic or delineated time scale. These energy levels ...
react directly to form
products Product may refer to: Business * Product (business), an item that serves as a solution to a specific consumer problem. * Product (project management), a deliverable or set of deliverables that contribute to a business solution Mathematics * Produ ...
in a single
reaction step A reaction step of a chemical reaction is defined as: ''"An elementary reaction, constituting one of the stages of a stepwise reaction in which a reaction intermediate (or, for the first step, the reactants) is converted into the next reaction int ...
and with a single
transition state In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked wi ...
. In practice, a reaction is assumed to be elementary if no
reaction intermediates In chemistry, a reaction intermediate or an intermediate is a molecular entity that is formed from the reactants (or preceding intermediates) but is consumed in further reactions in stepwise chemical reactions that contain multiple elementary s ...
have been detected or need to be postulated to describe the reaction on a molecular scale. An apparently elementary reaction may be in fact a
stepwise reaction In chemistry, a stepwise reaction (also called an overall reaction, complex reaction, and multistep reaction, among others) is a chemical reaction with one or more reaction intermediates, which by definition involves at least two consecutive elem ...
, i.e. a complicated sequence of chemical reactions, with reaction intermediates of variable lifetimes. In a
unimolecular In chemistry, molecularity is the number of molecules that come together to react in an elementary (single-step) reactionAtkins, P.; de Paula, J. Physical Chemistry. Oxford University Press, 2014 and is equal to the sum of stoichiometric coeffici ...
elementary reaction, a
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
dissociates or isomerises to form the products(s) :\mbox \rightarrow \mbox At constant temperature, the rate of such a reaction is proportional to the concentration of the species :\frac=-k
mbox Mbox is a generic term for a family of related file formats used for holding collections of email messages. It was first implemented in Research Unix, Fifth Edition Unix. All messages in an mbox mailbox are concatenated and stored as plain text ...
In a
bimolecular In chemistry, molecularity is the number of molecules that come together to react in an elementary (single-step) reactionAtkins, P.; de Paula, J. Physical Chemistry. Oxford University Press, 2014 and is equal to the sum of stoichiometric coeffici ...
elementary reaction, two
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, and ...
s, molecules,
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s or
radical Radical may refer to: Politics and ideology Politics *Radical politics, the political intent of fundamental societal change *Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe and ...
s, and , react together to form the product(s) :\mbox \rightarrow \mbox The rate of such a reaction, at constant temperature, is proportional to the product of the concentrations of the species and :\frac=\frac=-k
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The rate expression for an elementary bimolecular reaction is sometimes referred to as the
Law of Mass Action In chemistry, the law of mass action is the proposition that the rate of the chemical reaction is directly proportional to the product of the activities or concentrations of the reactants. It explains and predicts behaviors of solutions in dy ...
as it was first proposed by Guldberg and
Waage Waage is a Norwegian surname. Notable people with the surname include: *Anita Waage (born 1971), Norwegian footballer *Elsa Waage (born 1959), Icelandic opera singer *Geir Waage (born 1967), Norwegian politician *Hilde Waage (born 1959), Norwegian ...
in 1864. An example of this type of reaction is a
cycloaddition In organic chemistry, a cycloaddition is a chemical reaction in which "two or more Unsaturated hydrocarbon, unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of th ...
reaction. This rate expression can be derived from first principles by using
collision theory Collision theory is a principle of chemistry used to predict the rates of chemical reactions. It states that when suitable particles of the reactant hit each other with correct orientation, only a certain amount of collisions result in a percept ...
for
ideal gases An ideal gas is a theoretical gas composed of many randomly moving point particles that are not subject to interparticle interactions. The ideal gas concept is useful because it obeys the ideal gas law, a simplified equation of state, and is a ...
. For the case of dilute fluids equivalent results have been obtained from simple probabilistic arguments. According to collision theory the probability of three chemical species reacting simultaneously with each other in a termolecular elementary reaction is negligible. Hence such termolecular reactions are commonly referred as non-elementary reactions and can be broken down into a more fundamental set of bimolecular reactions,Aris, R. and Gray, P. and Scott, SK, Modelling cubic autocatalysis by successive bimolecular steps, Chemical Engineering Science 43', 207--211 (1988) in agreement with the law of mass action. It is not always possible to derive overall reaction schemes, but solutions based on
rate equation In chemistry, the rate law or rate equation for a reaction is an equation that links the initial or forward reaction rate with the concentrations or pressures of the reactants and constant parameters (normally rate coefficients and partial reacti ...
s are often possible in terms of
steady-state In systems theory, a system or a process is in a steady state if the variables (called state variables) which define the behavior of the system or the process are unchanging in time. In continuous time, this means that for those properties ''p'' ...
or Michaelis-Menten approximations.


Notes

{{Reaction mechanisms Chemical kinetics Physical chemistry