Under standard temperature and pressure, fluorine is a yellow gas. It is a halogen and is found at the top of its group on the periodic table. It is an extremely reactive element.
Because it lacks an electron in its outer shell, fluorine reacts aggressively with just about any material in an effort to grab an electron. It explodes in water and corrodes glass, ceramic and metal. This makes fluorine both difficult to handle and difficult to store. Its reactivity is also why it is never found in a free state, but as a fluoride. Fluorides can be remarkably stable and can be beneficial.