Fluoride is the anion F−, the reduced form of fluorine. Both organic and inorganic compounds containing the element fluorine are sometimes called fluorides. Fluoride, like other halides, is a monovalent ion (−1 charge). Its compounds often have properties that are distinct relative to other halides. Structurally, and to some extent chemically, the fluoride ion resembles the hydroxide ion.
Fluorine-containing compounds range from potent toxins such as sarin to life-saving pharmaceuticals such as efavirenz and from refractory materials such as calcium fluoride to the highly reactive sulfur tetrafluoride. The range of fluorine-containing compounds is considerable as fluorine is capable of forming compounds with all the elements except helium and neon. Compounds containing fluoride anions and in many cases those containing covalent bonds to fluorine are called fluorides.