Fluorine (Latin: fluere, meaning "to flow"), is the chemical element with the symbol F and atomic number 9. It is a nonmetallic, diatomic gas that is a trace element and member of the halogen family. Pure fluorine (F2) is a corrosive, poisonous, pale yellowish brown gas that is a powerful oxidizing agent. It is the most reactive and electronegative of all the elements (4.0), and readily forms compounds with most other elements. Fluorine even combines with the noble gases, krypton, xenon, and radon. Even in dark, cool conditions, fluorine reacts explosively with hydrogen. It is so reactive that glass, metals, and even water, as well as other substances, burn with a bright flame in a jet of fluorine gas. It is far too reactive to be found in elemental form and has such an affinity for most elements, including silicon, that it can neither be prepared nor be kept in ordinary glass vessels. Instead, it must be kept in specialized quartz tubes lined with a very thin layer of fluorocarbons. In moist air it reacts with water to form also-dangerous hydrofluoric acid. Elemental fluorine is a powerful oxidizer which can cause organic material, combustibles, or other flammable materials to ignite. Both elemental fluorine and fluoride ions are highly toxic and must be handled with great care and any contact with skin and eyes should be strictly avoided. Physiologically, fluorine. exists as an ion in the body. When it is a free element, fluorine has a characteristic pungent odor that is detectable in concentrations as low as 20 nL/L. Fluorine is used in dentistry as flouride (Fluorides) to prevent dental caries. Sodium and stannous salts of fluorine are commonly used in dentifrices. Contact of exposed skin with HF (hydrofluoric acid) solutions posses one of the most extreme and insidious industrial threats-- one which is exacerbated by the fact that HF damages nerves in such a way as to make such burns initially painless. The HF molecule is capable of rapidly migrating through lipid layers of cells which would ordinarily stop an ionized acid, and the burns are typically deep. HF may react with calcium, permanently damaging the bone. More seriously, reaction with the body's calcium can cause cardiac arrhythmias, followed by cardiac arrest brought on by sudden chemical changes within the body. These cannot always be prevented with local or intravenous injection of calcium salts. HF spills over just 2.5% of the body's surface area, despite copious immediate washing, have been fatal If the patient survives, HF burns typically produce open wounds of an especially slow-healing nature. Fluorine in the form of fluorspar (also called fluorite) (calcium fluoride) was described in 1530 by Georgius Agricola for its use as a flux , which is a substance that is used to promote the fusion of metals or minerals. In 1670 Schwanhard found that glass was etched when it was exposed to fluorspar that was treated with acid. Karl Scheele and many later researchers, including Humphry Davy, Gay-Lussac, Antoine Lavoisier, and Louis Thenard all would experiment with hydrofluoric acid, easily obtained by treating calcium fluoride (fluorspar) with concentrated sulfuric acid.