Halomethane the interaction of chlorine sources with various carbon compounds. The biosyntheses of these halomethanes are catalyzed by the chloroperoxidase and bromoperoxidase enzymes, respectively. An idealized equation is: Halons are usually defined as hydrocarbons where the hydrogen atoms have been replaced by bromine, along with other halogens. They are referred to by a system of code numbers similar to (but simpler than) the system used for freons. The first digit specifies the number of carbon atoms in the molecule, the second is the number of fluorine atoms, the third is the chlorine atoms, and the fourth is the number of bromine
Fluorine other elements from carbon to magnesium are twenty or more times as common. This is because stellar nucleosynthesis processes bypass fluorine, and any fluorine atoms otherwise created have high nuclear cross sections, allowing further fusion with hydrogen or helium to generate oxygen or neon respectively. Beyond this transient existence, three explanations have been proposed for the presence of fluorine: Fluorine is the thirteenth most common element in Earth's crust at 600–700 ppm (parts per million) by mass. Elemental fluorine in Earth's atmosphere would easily react with atmospheric water vapor, precluding its natural occurrence; it is found only in combined mineral