No textbook talks about Astatine that much, and neither does an average high school teacher, but there is a reason it exists... or is there?
Astatine’s name screams “unstable”: quite literally, because that is what its root word, astastos, means in Greek. And that instability is reflected in the fact that one can’t find more than 20 grams of astatine on the Earth at any given time. Even more bizarre is the fact that some believe astatine was discovered by a guinea pig, although not all by itself. A guinea pig out of all things might have achieved something that, by 1940, 3 reputed scientists from 3 different countries had failed to.
<a href="https://www.freepik.com/free-photo/cute-guinea-pig-green-grass-garden_9129336.htm#query=guinea%20pig&position=4&from_view=keyword">Image by devmaryna</a> on Freepik
When scientists at UC Berkeley injected a lab sample of what they hoped was a new element into a guinea pig, they assumed it would accumulate in the thyroid gland of the animal much like its halogenic predecessor, Iodine. And rightly so, because that’s how the discovery of element number 85 came to be. Can’t underestimate a guinea pig, can you?
It has been 83 years since astatine exposed itself to man, but it has remained so elusive ever since that no one has been able to gather a pinch of pure astatine in one place. Reason? Astatine’s most stable isotope has a half-life of 8.1 hours, and that is leagues ahead of Francium, the second-rarest element on earth, whose most stable isotope lives on for about 22 minutes before it is halved. What kills astatine is nothing more than its suicidal nature: it is so radioactive that it gets vaporised by its own heat. That is also the reason why I had to use an image of a guinea pig to start and not astatine.
Notwithstanding all the shenanigans of Astatine, it does have a few eye-catching applications, mainly in radiation therapy for cancer treatment. While the At-210 isotope does decay into potentially fatal Polonium toxins, the At-211 is very promising in radiation therapy. It is an alpha-emitter, meaning it emits alpha particles as it decays, but its short half-life is a glaring advantage it has over other alpha emitters, not only because it gets the job done quicker, but also because it is slightly easier to track.
Image Credit: American Chemical and Engineering News
Another feature of its that truly sets Astatine apart from other emitters is the fact that it fires only one alpha-particle, which facilitates precision targeting of cancer cells without damaging the surrounding cells. The decay chain almost always ends in the formation of harmless Pb-207 or Bi-207. But the real problem lies in producing the astatine. Astatine’s short half-life is both a boon and a curse; separating the At-211 isotopes from the decayed products of the chain eats up a chunk of that time, roughly around 1h20m. It takes around another 5h30m to sterilise it and attach it to an antibody, and we are already about 7h in, and half of it is already gone.
Unlike some other alpha emitters produced by a natural decay chain, At-211 needs to be produced via a cyclotron by bombarding Bi-209 molecules. The problem lies in Bismuth’s low melting point, meaning one would need to provide enough simultaneous cooling to the sample to prevent it from melting. Scientists would also need to carefully monitor and calibrate the energies of the beams of the cyclotron, as any slight variation may produce undesirable At-210. Even more burdensome would be having access to a cyclotron with the desired specifications, as one does cost quite a lot. We’re talking millions.
Okay forget radiation therapy, Astatine must at least form a powerful acid, right? In theory, Hydroastatic acid, HAt, or simply Hydrogen Astatide, should have a very weak bond and should liberate protons with ease. In practice, not so much: the electronegativities of Hydrogen and Astatine are very similar(you can google them and it would show the same number), and this means that the possibility of formation of an At+ ion and the hydride(H-) ion is just as likely as proton removal. That is, in ideal conditions,
2HAt ———> H⁺ + At⁻ + H⁻ + At⁺ ———> H2 + At2
This possibility is very real since the existence of At+ cation has been confirmed. For now, Hydroiodic acid remains the king of diatomic acids, and the Why is Astatine remains a mystery.
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