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
Biological aspects of fluorine Biological aspects of fluorine Biological aspects of fluorine describes the effects of fluorine-containing compounds with life. Fluorine (F) itself is very rare in everyday life, but hundreds of fluorine-containing compounds occur as minerals, medicines, pesticides, and materials. Naturally occurring organofluorine compounds are extremely rare. Nonetheless, twenty percent of all commercialized pharmaceuticals contain fluorine, examples being Lipitor and Prozac. Manmade fluorinated compounds have also played a role in several noteworthy environmental concerns. Chlorofluorocarbons, once major components of numerous commercial aerosol products, have proven damaging to Earth's ozone layer and resulted in the wide-reaching Montreal Protocol (though in truth the chlorine in