Astatine ($At$)
The rarest naturally occurring element on Earth—a highly radioactive, deeply mysterious halogen that vanishes almost as soon as it is created.
Astatine holds a legendary status among chemists. Its name is derived from the Greek word astatos, meaning "unstable." It is estimated that at any given moment, there is less than 30 grams of astatine in the entire Earth's crust. It exists naturally only as a fleeting intermediate in the decay chains of heavier elements like uranium and thorium.
Occupying Group 17 and Period 6, astatine is the heaviest known halogen (discounting the superheavy synthetic element tennessine). It was officially synthesized and discovered in 1940 by Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè at the University of California, Berkeley. Because of its extreme radioactivity and incredibly short half-life, astatine has never been collected in a macroscopic quantity.
Atomic & Radioactive Properties
Astatine has no stable isotopes. Its most stable isotope, Astatine-210, has a half-life of just 8.1 hours. Because it decays so rapidly, its physical properties must be inferred through theoretical calculations and observations of trace quantities.
| Property | Value |
|---|---|
| Atomic Number | 85 |
| Standard Atomic Weight | [210] (Most stable isotope) |
| Electron Configuration | $[Xe] 4f^{14} 5d^{10} 6s^2 6p^5$ |
| Most Stable Isotope | 210At (Half-life: 8.1 hours) |
| Common Oxidation States | -1, +1, +3, +5, +7 |
| Phase at STP (Predicted) | Solid (Dark, metallic) |
The Macroscopic Mystery
What does it look like?
No human has ever seen a chunk of astatine. If a piece large enough to be seen with the naked eye were ever assembled, it would instantly vaporize itself due to the intense heat generated by its own radioactivity.
Chemists debate whether astatine behaves more like a non-metal halogen (like iodine) or a true metal. Current relativistic calculations suggest that bulk astatine would likely be a dark, metallic solid, functioning as a semiconductor or possibly even a true metal, differing significantly from the diatomic, non-metallic nature of iodine.
Chemical Reactivity: The Final Halogen
Despite the challenges in studying it, scientists have mapped out astatine's chemistry using highly diluted tracer techniques. As expected from periodic trends, it is the least electronegative and least reactive of the halogens.
1. Formation of Astatides
Like its lighter siblings, astatine can gain an electron to form the astatide ion ($At^-$). It reacts with hydrogen to form hydrogen astatide, a highly unstable and acidic gas.
2. Interhalogen Compounds
Because astatine has a lower electronegativity than other halogens, it can form compounds where it acts as the positive cation. It readily reacts with iodine, bromine, and chlorine to form interhalogen compounds.
Medicine: Targeted Alpha Therapy (TAT)
You might wonder if such a rare and dangerous element has any practical use. Surprisingly, astatine is at the cutting edge of cancer research. The isotope Astatine-211 ($^{211}At$) decays primarily via alpha emission and has a half-life of 7.2 hours.
In Targeted Alpha Therapy (TAT), $^{211}At$ is chemically attached to an antibody designed to seek out cancer cells. Once it attaches to the tumor, it emits heavy alpha particles. These particles carry massive energy but only travel a few cell diameters. This means the astatine destroys the cancer cells with extreme prejudice while leaving the surrounding healthy tissue completely unharmed.
Artificial Synthesis
To produce astatine for medical research, physicists must create it on demand. This is done by bombarding a target of Bismuth-209 with high-energy alpha particles inside a cyclotron.
The resulting astatine must be rapidly separated from the bismuth target through distillation and immediately shipped to the hospital for treatment before it decays away.
Conclusion of the Halogens
Astatine concludes the classical halogen group. By comparing fluorine (the ultimate reactive gas) to astatine (a heavy, unstable, likely metallic solid), we witness one of the most dramatic demonstrations of periodic trends. Astatine reminds us that at the extreme edges of the periodic table, the rules of classic chemistry begin to blur into the realm of nuclear physics.
This is the eighty-fifth part of our "Elements and Their Properties" series. We have reached the edge of stability in the p-block! To master the concepts of radioactive decay chains and interhalogen compounds, visit our Success Blueprint.
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