Astatine

Astatine is a chemical element with the symbol At and atomic number 85. It is the rarest naturally occurring element found in the Earth's crust. It does not exist in large amounts on its own. Instead, it appears only as the decay product of much heavier elements.

This means as heavier atoms break down, they eventually turn into astatine. Because it is so rare, scientists find it very difficult to study in large amounts.

The primary challenge with astatine is its extreme radioactivity. All known isotopes of astatine are very short-lived. The most stable version is astatine-210, which has a half-life of only 8.1 hours. A half-life is the time it takes for half of a radioactive sample to decay. Most other isotopes are even more unstable, lasting only seconds or less. This rapid decay releases an intense amount of heat. If a person could hold a macroscopic, or large, solid piece of astatine, the heat from its own radioactivity would immediately vaporize it into a gas.

Scientists have had to estimate the physical properties of astatine because they cannot see a solid sample. They use its position on the periodic table to make educated guesses. Astatine is part of the halogen group, which includes fluorine, chlorine, bromine, and iodine. In this group, elements tend to get darker as they get heavier. Since iodine is dark gray or violet, astatine is predicted to be a dark or black solid. However, astatine also sits near the dividing line between metals and nonmetals. This suggests it might behave as a metalloid or even a metal. Some researchers believe it could be a semiconductor or a superconductor.

The history of astatine began with its first synthesis in 1940. Three scientists, Dale R. Corson, Kenneth Ross MacKenzie, and Emilio G. Segrè, created it at the University of California, Berkeley.

They named the element after the Ancient Greek word *astatos*, which means unstable. Since that discovery, researchers have identified four naturally occurring isotopes. However, there is less than one gram of astatine present in the Earth's crust at any given time. To study it, scientists usually produce it artificially. They do this by bombarding bismuth-209 with alpha particles.

Chemical studies of astatine are complicated by its extremely low concentrations. Experiments often use tracer studies on very dilute solutions. These solutions are often less than 10⁻¹⁰ mol·L⁻¹. To make laboratory techniques like filtration possible, scientists use iodine as a carrier. The iodine helps the tiny amounts of astatine stay in place during the experiment. Astatine can form many different types of chemical bonds. It can bond with elements like boron, carbon, and nitrogen. It can also react with other halogens to form interhalogen compounds like AtI, AtBr, or AtCl.

Astatine shows both nonmetal and metallic characteristics. As a halogen, it can form anions, which are negatively charged particles. It can also act like a metal by plating onto a cathode. In some cases, its behavior is similar to silver. For example, it can form complexes with EDTA, which is a metal chelating agent. It can also be used in antibody radiolabeling. One interesting prediction involves its bond with hydrogen. While officially called hydrogen astatide, some suggest it should be called astatine hydride. This is because the electronegativity of astatine is lower than that of hydrogen.

Understanding astatine helps scientists learn about the boundaries of the periodic table. It provides a look at how elements behave when they are right on the edge of being metals. Its unique properties, such as its high radioactivity and its place in the halogen group, connect it to many different fields of science. From nuclear physics to medicinal chemistry, astatine remains one of the most mysterious elements in existence.