Bromine structure. The halogens darken in colour as the group is descended: fluorine is a very pale yellow gas, chlorine is greenish-yellow, and bromine is a reddish-brown volatile liquid that melts at −7.2 °C and boils at 58.8 °C. (Iodine is a shiny black solid.) This trend occurs because the wavelengths of visible light absorbed by the halogens increase down the group. Specifically, the colour of a halogen, such as bromine, results from the electron transition between the highest occupied antibonding "π" molecular orbital and the lowest vacant antibonding "σ" molecular orbital. The colour fades at low temperatures, so that solid
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