Dimer (chemistry) dimerise to give the dimer "A-A". An example is a diaminocarbene, which dimerise to give a tetraaminoethylene: Carbenes are highly reactive and readily form bonds. 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: Many nonmetallic elements occur as dimers: hydrogen, nitrogen, oxygen, the halogens, i.e. fluorine, chlorine, bromine and iodine. Noble gases can form dimers linked by van der Waals bonds, for example dihelium or diargon. Mercury occurs as a mercury(I) cation (Hg), formally a dimeric
Chlorine nuclide in a ratio of about (7–10) × 10 to 1 with stable chlorine isotopes: it is produced in the atmosphere by spallation of Ar by interactions with cosmic ray protons. In the top meter of the lithosphere, Cl is generated primarily by thermal neutron activation of Cl and spallation of K and Ca. In the subsurface environment, muon capture by Ca becomes more important as a way to generate Cl. Chlorine is intermediate in reactivity between fluorine and bromine, and is one of the most reactive elements. Chlorine is a weaker oxidising agent than fluorine but a stronger one