Fluorine

Everyone takes fluoride in toothpaste and drinking water for granted. There is nothing wrong with fluoride, right? Unless you believe in conspiracies, that is. And some of that has to do with the way fluorine works. It can be dangerous because this is the periodic table's most reactive element. When fluorine gas is exposed to most metals and even water, it produces an incredibly intense flame.

Fluorine only gets along quite well with four other elements. You get hydrofluoric acid if you expose it to hydrogen. That is very difficult to keep because, in contrast to many acids, it will dissolve glass. Even diamonds can be harmed by fluorine.

Fluorine interacts to everything, so it doesn't appear in nature in its pure form. It must be purified from other substances instead. The abundance of fluorine on Earth is maybe one of its strangest properties. It forms in the stars in roughly 400 parts per billion, making it the 13th most frequent element in the Earth's crust yet nearly unheard of in the rest of the cosmos.