Editing Chemical reaction (section)

==Elementary reactions==
The [[elementary reaction]] is the smallest division into which a chemical reaction can be decomposed, it has no intermediate products.<ref>{{GoldBookRef|title=elementary reaction|file=E02035}}</ref> Most experimentally observed reactions are built up from many elementary reactions that occur in parallel or sequentially. The actual sequence of the individual elementary reactions is known as [[reaction mechanism]]. An elementary reaction involves a few molecules, usually one or two, because of the low probability for several molecules to meet at a certain time.<ref>{{cite encyclopedia | author = Frenking, Gernot | contribution = Elementarreaktionen | title = Römpp Chemie-Lexikon | publisher = [[Thieme Medical Publishers|Thieme]] | year = 2006}}</ref>

[[File:Azobenzene isomerization de.svg|thumb|300px|Isomerization of [[azobenzene]], induced by light (hν) or heat (Δ)]]
The most important elementary reactions are unimolecular and bimolecular reactions. Only one molecule is involved in a unimolecular reaction; it is transformed by isomerization or a [[dissociation (chemistry)|dissociation]] into one or more other molecules. Such reactions require the addition of energy in the form of heat or light. A typical example of a unimolecular reaction is the [[cis–trans isomerism|cis–trans]] [[isomerization]], in which the cis-form of a compound converts to the trans-form or vice versa.<ref name=rh/>

In a typical [[dissociation (chemistry)|dissociation]] reaction, a bond in a molecule splits ('''ruptures''') resulting in two molecular fragments. The splitting can be [[homolysis (chemistry)|homolytic]] or [[Heterolysis (chemistry)|heterolytic]]. In the first case, the bond is divided so that each product retains an electron and becomes a neutral [[Radical (chemistry)|radical]]. In the second case, both electrons of the chemical bond remain with one of the products, resulting in charged [[ion]]s. Dissociation plays an important role in triggering [[chain reaction]]s, such as [[Oxyhydrogen|hydrogen–oxygen]] or [[polymerization]] reactions.

:<chem>AB -> A + B</chem>
: <small> Dissociation of a molecule AB into fragments A and B </small>

For [[bimolecular]] reactions, two molecules collide and react with each other. Their merger is called [[chemical synthesis]] or an [[addition reaction]].
:<chem>A + B -> AB</chem>

Another possibility is that only a portion of one molecule is transferred to the other molecule. This type of reaction occurs, for example, in [[redox]] and acid-base reactions. In redox reactions, the transferred particle is an electron, whereas in acid-base reactions it is a proton. This type of reaction is also called [[Salt metathesis reaction|metathesis]].
:<chem>HA + B -> A + HB</chem>
for example
:<chem>NaCl + AgNO3 -> NaNO3 + AgCl(v)</chem>