When we think of the halogens, F, Cl, Br and I are generally those that spring to mind. Yet there is, of course, another one — astatine. In contrast to the first four, ubiquitous on earth and which serve in numerous reactions, astatine is rare and has remained a bit of a mystery. It is the topic of this month’s ‘in your element’ article (subscription required), written by Scott Wilbur from the Department of Radiation Oncology at the University of Washington.
As its name reflects — astatos is the Greek word for unstable — it is radioactive. All of the known isotopes of astatine are radioactive, the longest-lived ones (210At and 211At) with half-lives of only 8.1 and 7.2 h, respectively. This does not facilitate chemical and physical characterization, in particular making it impossible to weigh and even observe element 85 in the conventional sense. Fortunately, these two isotopes can be produced by irradiation of bismuth targets — only in very small amounts, but sufficient for some research nonetheless.
The field in which astatine has attracted the most attention is medicine — but only the 211 isotope; its 210 counterpart is definitely unsuitable as it decays into polonium-210, a species that made the news a few years ago through the poisoning of Alexander Litvinenko. Actually, 211At is one of the rare a-emitters to be investigated for medical applications; they usually wreak havoc in internal organs. I’ll let you go to the article to read about its potential medical uses — as well as the challenges in investigating it, including how difficult it is to even determine whether or not it is released from a carrier molecule.
In some other ways, astatine behaves very much like other halogens and undergoes electrophilic and nucleophilic reactions. I wouldn’t recommend trading halogens for astatine in any of your up-coming reactions just yet; reproducibility can be an issue (this is not overly surprising considering you may only have about 10-13 to a maximum of 10-8 grams of astatine in any given sample and this may be a smaller amount than any trace impurities!). In any case, we don’t know nearly enough about astatine’s physical and chemical properties — but if you like working with minute amounts of decaying species, give it some thought!
Anne Pichon (Associate Editor, Nature Chemistry)