Flourine is the most chemically aggressive element. In pure form it is a pale yellow-green F₂ Gas. It is extremely toxic and reacts with nearly everything, in most cases very violently. At contact with water,it forms the very caustic hydrofluoric acid, HF. Its salts(fluorides),especially fluorite(Calcium Fluoride,CaF₂) frequently occure in nature as minerals. Fluoride is needed for bones and teeth, but quickly becomes poisonous if the dose is too high.