Astatine At

Discovery

Mendeleev predicted an element below iodine and called it eka-iodine. In the 1930s several discovery claims, including "alabamine" and "helvetium", appeared but none survived scrutiny.

In 1940 Dale Corson, Kenneth MacKenzie and Emilio Segrè at the University of California, Berkeley, made element 85 by bombarding bismuth-209 with alpha particles from a cyclotron. Segrè had earlier co-discovered technetium (43). In 1943 Berta Karlik and Traude Bernert in Austria showed that astatine also occurs naturally as a short-lived member of radioactive decay chains.

Origin of the name

The name comes from the Greek astatos, meaning "unstable", a fitting description of an element whose isotopes all decay quickly. The ending -ine follows the pattern of the other halogens: fluorine, chlorine, bromine and iodine.

Main uses

Only minute amounts of astatine can be produced, so it has no industrial uses. Its promise lies in medicine and research:

• Targeted alpha therapy: astatine-211 has a half-life of about 7.2 hours, long enough to ship to hospitals, and emits alpha particles that can kill cancer cells while sparing more of the surrounding tissue. Clinical studies are attaching it to antibodies and other targeting molecules for cancers such as thyroid cancer and leukaemia.
• Fundamental chemistry: as the heaviest halogen studied experimentally, it shows how relativistic effects change chemical behaviour.

Isotopes

All astatine isotopes are radioactive. Even the longest-lived, astatine-210, has a half-life of only about 8.1 hours. Medical astatine-211 is made in cyclotrons by bombarding bismuth-209 with alpha particles. In nature, isotopes such as astatine-218 and -219 appear fleetingly on minor branches of the uranium and thorium decay chains.

In everyday life

At any given moment the Earth's entire crust is estimated to contain only around a gram of astatine. A visible lump would probably vaporise at once from the heat of its own radioactivity. A 2013 theoretical study suggested that condensed astatine might be metallic, unlike the other halogens, but this has not been tested experimentally. You will never encounter astatine in everyday life.