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Fluorine (from L. fluere, meaning to flow), is the chemical element in the periodic table that has the symbol F and atomic number 9. Atomic fluorine is univalent and is the most chemically reactive and electronegative of all the elements. In its pure form, it is a poisonous, pale, yellow-green gas, with chemical formula F2. Like other halogens, molecular fluorine is highly dangerous; it causes severe chemical burns on contact with skin.
Fluorides are compounds that combine fluoride with some positively charged counterpart. They often consist of ions. Fluorine compounds with metals are among the most stable of salts.
Sodium fluoride has been used as an insecticide, especially against cockroaches. It is also often added to toothpaste and, somewhat controversially, to municipal water supplies to prevent dental cavities.
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