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Fluorine (IPA: , , meaning "to flow"), is the chemical element with 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 elementally isolated (pure) form, fluorine is a poisonous, pale, yellow-green gas, with chemical formula F<sub>2</sub>. Like other halogens, molecular fluorine is highly dangerous; it causes severe chemical burns on contact with skin.