Astatine

On the periodic table, the space below iodine, reserved for element 85, sat empty for decades. Many scientists claimed to have found and isolated the elusive element, but none of these results were repeatable. In 1940, D.R. Corson, K.R. MacKenzie, and Emilio Segrè were able to create element 85 in a particle accelerator (called a cyclotron) at the University of California, Berkeley. The team bombarded bismuth-209 with alpha particles, causing the bismuth to emit two neutrons and, in doing so, become astatine-211. They chose the name astatine for their discovery, inspired by the Greek word astatos, meaning “unstable.”

Aptly named, astatine is highly radioactive, meaning it decays rapidly. The scientific community was reluctant at the time of its discovery to accept synthetically made, highly radioactive isotopes as legitimate elements. Fortunately for astatine, which has no stable isotopes, this changed in 1946 when the element was formally recognized.

The most stable astatine isotope, synthetically produced astatine-210, has a half-life of only 8.1 hours. Most other isotopes of astatine have half-lives of a few seconds or less, decaying rapidly into polonium, bismuth, or radon. Because of its short half-life, the properties of astatine are difficult to discern. A weighable sample of the pure element is nearly impossible to assemble for study. Any visible portion of astatine would immediately vaporize itself because of its extreme radioactivity. Therefore, physical properties of this element are mostly speculative, either derived from theory or extrapolated from the properties of other halogens (those elements in column 9 of the periodic table).

Mass spectrometers have been used to confirm that astatine behaves chemically very much like other halogens, particularly iodine (although astatine is thought to be more metallic). Like iodine, astatine accumulates in the thyroid gland.

The only isotope with an identified potential for use is astatine-211. Research in the field of nuclear medicine has shown that its short half-life and limited penetration through tissue offer advantages over current radiation therapies (such as iodine-131) for cancer treatment when a tumor is very small or in close proximity to important non-cancerous tissue.

Astatine is known to form compounds but is the least reactive of the halogens. It is most often studied in combination with iodine to ensure a sufficient quantity of substance to perform tests.

Astatine is continuously forming in the Earth’s crust as a decay product of the radioactive elements uranium and thorium. Its half-life is so short (a matter of seconds for most isotopes), that it does not accumulate, decaying rapidly into other elements. It is present on Earth in quantities of less than 30 grams at any given time. The isotopes astatine-209, -210, and -211 can be produced in small quantities in a cyclotron by bombarding bismuth with energetic alpha particles.