Very Little Astatine

Question

Astatine is the penultimate halogen in the periodic table with atomic number 85. It is very much radioactive and would vaporize itself by its own radioactivity before being collected. But there is something that Wikipedia says which is difficult for me to comprehend. It says

Astatine is the rarest naturally occurring element. The total amount of astatine in the Earth's crust (quoted mass $2.36 × 10^{25}$ grams) is estimated by some to be less than one gram at any given time. Other sources estimate the amount of ephemeral astatine, present on earth at any given moment, to be up to one ounce (about 28 grams).

My question is how can we even predict that the amount of astatine on the planet at any time must be less than 1 gram or about 28 grams if we haven't ever had any samples to investigate the element? Or are there any indirect means to do the same?

Answer 1

It's true that astatine is radioactive and it will vaporize by its own radiation. Not to mention its very low half-life (8.1 hours), meaning it will lose half of its mass before you even try to detect but there are novel ways to detect and measure astatine:

• by using its ionization potential (the amount of energy needed to remove one electron from an atom, turning it into an ion or a charged particle).

Physicists at CERN's ISOLDE (Isotope Separator On Line-Detector) Radioactive Ion Beam facility created artificial isotopes of astatine (atoms with different numbers of neutrons than those occurring in nature) by shooting beams of energetic protons at a target of uranium (which has 92 protons and electrons). The collisions created a shower of new particles, some of which were astatine. The physicists then shined laser beams of varying wavelengths at the atoms to ionize them. By isolating the astatine ions, and checking which wavelength of laser had created them, the researchers determined astatine's ionization potential to be 9.31751 electronvolts (the ionization potential of hydrogen, for example, is 13.6 electronvolts) [Ref. 1]

• By bombarding a heavy element with some light mass (e.g. bombarding bismuth isotope with alpha particles using a cyclotron confirmed the existence of Astatine-211 --> Quantity obtained: 0.001 mg). $$\ce{^{209}_{83}Bi + ^{4}_{2}He -> ^{211}_{85}At + 2^{1}_{0}n}$$
• through radioactive decay chain. Astatine is one of the isotopes formed from uranium decay, plutonium decay etc.

Through this detection, scientist were able to estimate how much astatine would be there at a specific time.

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Little bit of historical info.:

Astatine was so hard to detect and produce that scientists who first created it in 1939 had to resort to a trick. They created a tiny bit of astatine within a sample of bismuth by bombarding the bismuth with particles from a cyclotron. They then fed the whole thing to a guinea pig. After a few hours of digestion the guinea pig’s iodine-hungry thyroid gland had filtered and concentrated the astatine. It remains the only element discovered by a nonhuman.

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Fun Fact: Its name comes from the Greek "astastos" meaning "unstable."

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Reference

1. https://www.scientificamerican.com/article/physicists-measure-earths-rarest-element/
2. https://www.azom.com/article.aspx?ArticleID=7907
3. https://www.sciencehistory.org/distillations/tiny-productions
4. https://www.npr.org/sections/krulwich/2013/07/12/201481293/the-hardest-thing-to-find-in-the-universe