Fluorine (Atomic Number: 9).
Fluorine appears as a pale yellow gas, which is so corrosive; it can even destroy borosilicateglass (The stuff chemistry flasks are made of).
Fluorine, being the most reactive and electronegative element in existence, is both highly corrosive and toxic. Once it does react, it bonds so strongly that the resulting molecule is very inert and non-reactive to anything else. Teflon for example, is fluorine bonded with carbon.
Fluorine is a halogen: One of five non-metal elements, which contain a single highly reactive electron (-1 charge) (Monovalent ion).