Pure fluorine is not generally available, since it is extremely dangerous, but fluorine compounds known as fluorides can be found in dental products and municipal water supplies, among other places. The atomic number of fluorine is nine, and the element belongs to the halogen group. It is the lightest of the halogen gases, and the most reactive naturally occurring element, due to its electronegative configuration. On the periodic table of elements, fluorine is identified with the symbol F. The element was first isolated in 1886, at great peril to the scientists who did so.