Astatine (At)
“The rarest natural element on Earth — only about 25 grams exist in the entire crust at any instant, and every atom is gone within hours.”
How its electrons are arranged
Computed from the aufbau principle, with known real-world exceptions applied automatically.
Characteristics
- Electron configuration [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵ — period 6, group 17, p-block, seven valence electrons: the heaviest halogen, with fluorine, chlorine, bromine and iodine as its seniors.
- Every known isotope is radioactive; the longest-lived, astatine-210, survives barely 8 hours — which is why no sample of it can ever be stored, weighed or admired.
- Only about 25 grams exist in Earth’s entire crust at any moment, brewed in tiny side-branches of the uranium and thorium decay chains and vanishing as fast as it forms.
- It breaks halogen rules: besides At⁻ it can form positive At⁺ ions like a metal, and like iodine it concentrates in the thyroid gland — the very trick medicine wants to borrow.
No human has ever seen a visible lump of astatine: theory says it would be a dark, near-black solid — the next step in the halogens’ colour march from pale-yellow fluorine to violet-black iodine. Any bulk sample would vaporise on the spot, cooked by the heat of its own radioactivity.
Where you meet it
✦ Uses
- Experimental thyroid radiotherapy: astatine-211 is being trialled against thyroid tumours, because its short-range alpha particles kill cancer cells while sparing more healthy tissue than iodine-131’s beta radiation.
- Radiopharmaceutical research: labs in Japan and the US produce At-211 in cyclotrons and hitch it to tumour-seeking antibodies — “targeted alpha therapy”, one of oncology’s liveliest frontiers.
- Fundamental chemistry of the heavy halogens: studying how the strangest halogen bonds sharpens the theories that describe the whole heavy end of the periodic table.
◍ Everyday examples
- Quiz champions meet astatine as “the rarest natural element” — there is less of it in the whole world than a spoonful of sugar.
- It hunts like iodine: just as iodine-131 treats overactive thyroids, experimental At-211 aims at the same gland, so every thyroid-scan story secretly leads here.
- Museum exhibits on the Berkeley cyclotron — the machine that made the first artificial elements — feature the 1940 astatine run.
History & name
Chemists chased element 85 for years, and false claimants claimed its seat at least twice — “alabamine” in Alabama (1931) and “helvetium” in Switzerland both proved errors. The true discovery came in 1940 at the University of California, Berkeley, where Dale Corson, Kenneth MacKenzie and Emilio Segrè fired alpha particles at bismuth in a cyclotron and minted astatine-211. The name came from the Greek astatos, “unstable”; soon afterwards the Austrian physicist Berta Karlik showed nature makes its own, catching natural astatine in the decay chains of uranium and thorium ores.