Over there, on the right-hand side of the periodic table, sit the halogens. And there, at the top, is an element which forms a pale-yellow, nearly invisible gas: fluorine.
You might be forgiven for thinking it doesn't sound that bad, after all, isn't it in toothpaste...? Actually, not exactly. The substance in toothpaste is sodium fluoride, which is what you get if you react sodium metal with fluorine. And to echo a point I've made before, although the tiny quantities in toothpaste and drinking water are entirely safe, consuming larger amounts would be quite dangerous. Don't worry, though, you'd probably have to eat about 13 tubes of toothpaste to poison yourself.
Fluorine gas, however, is a whole other matter. It's been described as "the tyrannosaurus rex of the periodic table," by chemistry professor Andrea Sella, because it "will react spontaneously with every other element except for helium, neon and argon."
It is indeed incredibly reactive; just the tiniest amount will fatally damage your liver, kidneys, eyes and skin in seconds. Thermal burns have been reported when it comes in contact with the skin due to the violent reaction between the skin and the gas.
Fluorine was first isolated in the late 19th century, and the list of chemists who ran afoul of it in the process is really quite impressive. For example, famous English chemist Humphry Davy - discoverer of many elements including calcium, chlorine and magnesium - was poisoned, but recovered. Belgian chemist Paulin Louyet and French chemist Jerome Nickles were less fortunate: they both died. Fluorine produces hydrofluoric acid (more about that in a moment) in contact with water, including the water in your lungs and on your skin, and this makes it truly deadly.
Fluorine smells a little like chlorine - which is far from innocuous itself. But if you think you're smelling fluorine gas, Sella's advice is to "run away as fast as you can."
Kat Day is a science blogger, writer and teacher living in Oxfordshire in the U.K. Her award-winning blog is called The Chronicle Flask, and she has also written articles for Sense About Science, Things We Don't Know and Nature Chemistry. When she's not writing or teaching she is usually trying to keep on top of important parenting skills such as negotiation, conflict resolution and always having the right coloured cup.