The Unstable One

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.