Astatine

Astatine was first synthesised in 1940 at the University of California, Berkeley, by Dale Corson, Kenneth MacKenzie and Emilio Segre, who created it by bombarding bismuth with alpha particles. It also occurs in nature in unimaginably tiny amounts as a fleeting product of decay chains. Its name comes from the Greek astatos, unstable.

Astatine is the rarest naturally occurring element on Earth, a radioactive halogen so short-lived that a visible, weighable sample has never been assembled. Formed fleetingly in the decay chains of heavier elements and otherwise made in accelerators, astatine has no uses beyond research, though its heavier isotopes are studied for targeted alpha-particle therapy against cancer, exploiting the very radioactivity that makes it so hard to study. Element 85 had its own share of premature announcements before the real discovery: researchers claimed alabamine in 1931 and helvetium in 1940, names now remembered only as historical footnotes once the true, fantastically short-lived astatine was properly synthesised at Berkeley. So little astatine has ever existed at any one time, an estimated total of well under a gram across the entire planet, that essentially everything known about its chemistry has been inferred from tracer-level experiments and theoretical calculation rather than from direct observation of a visible sample, a genuinely unusual position for a naturally occurring element to hold in modern chemistry.

Astatine's name, astatos, the unstable, is a plain statement of its defining trait: its longest-lived isotope survives only about eight hours, and every form of the element decays too quickly for any macroscopic quantity to exist. It is estimated that the entire crust of the Earth contains less than a teaspoon of astatine at any moment, continuously created and destroyed within decay chains. The element has no mythological, alchemical or cultural tradition of any kind, for it was never known to the pre-modern world and can barely be said to exist in bulk even now; its whole identity is scientific, and its Greek name honestly records the impermanence that keeps it forever just beyond reach of ordinary chemistry. It is chiefly of interest to nuclear medicine, where a heavy isotope is studied for targeted cancer therapy.

Producing even the tiny quantities of astatine-211 needed for cancer-therapy research requires a cyclotron capable of the specific alpha-particle bombardment of bismuth that generates it, a capability only a handful of research institutions worldwide possess, a major practical reason astatine-based therapies remain confined to early-stage clinical trials rather than any wider medical use decades after the element's radioactive potential was first recognised.