Editing Dimerization (section)

== Covalent dimers ==
[[File: Dicyclopentadiene structure.svg|thumb|right|100px|class=skin-invert-image|The dimerization of [[cyclopentadiene]] gives dicyclopentadiene, although this might not be readily apparent on initial inspection.  This dimerization is reversible.]]

[[Molecular]] dimers are often formed by the reaction of two identical compounds e.g.: {{chem2|2A -> A\sA}}. In this example, [[monomer]] "A" is said to dimerize to give the dimer "{{chem2|A\sA}}".

[[Dicyclopentadiene]] is an asymmetrical dimer of two [[cyclopentadiene]] molecules that have reacted in a [[Diels-Alder reaction]] to give the product. Upon heating, it "cracks" (undergoes a retro-Diels-Alder reaction) to give identical monomers:
:<chem>C10H12 ->  2  C5H6</chem>

Many nonmetallic elements occur as dimers: [[hydrogen]], [[nitrogen]], [[oxygen]], and the [[halogen]]s [[fluorine]], [[chlorine]], [[bromine]] and [[iodine]]. Some metals form a proportion of dimers in their vapour phase: [[dilithium]] ({{chem2|Li2}}), [[disodium]] ({{chem2|Na2}}), [[dipotassium]] ({{chem2|K2}}), [[dirubidium]] ({{chem2|Rb2}}) and [[dicaesium]] ({{chem2|Cs2}}). Such elemental dimers are [[homonuclear molecule|homonuclear]] [[diatomic molecule]]s.