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in your element
Enigmatic astatine
D. Scott Wilbur points out the difficulty in studying the transient element astatine, and the need to understand its basic chemical nature to help in the development of targeted radiotherapy agents.
Since the discovery of astatineover 70years ago1, many of its characteristics have remained elusive.
Unlike the other halogens, abundant and ubiquitous in nature, astatine is one ofthe rarest of all elements. This arises from the fact that it has no stable isotopes; the longest lived of its 32 known radioisotopes,
210At, has a half-life of only 8.1hours.
The rarity and radioactive nature of element 85 lends to its mystery, as it cannot be observed or weighed in a conventional sense. Even its colour is unknown; based on increasingly dark colours for halogens from uorine to iodine, however, black seems a logical guess.
The rarity of this radioactive element is reected in its name, derived from the Greek word (astatos) meaning unstable2. What little astatine is present in nature comes from the decay of heavy radioactive elements found in the Earthscrust. The total amount of natural astatine at any given time hasbeen estimated to be betweena few hundred milligrams3 and30g. In any case, naturallyoccurring astatine isotopesare too unstable, and wouldbe too difficult to obtain,for characterization.
Fortunately, the twolongest-lived isotopes 210At and 211At...