Unknown element would be metallic and radioactive

The largest amount of astatine ever created is 0.05 micrograms, which isn't very much at all, says chemist Roald Hoffman, though that it is still around 140,000 billion atoms. The researchers plan to keep exploring some of the fundamental mysteries in the periodic table. (Credit: Duncan C/Flickr)

Scientists have predicted what the rare element astatine would look like in its condensed form.

With a maximum half-life of just eight hours, astatine is found in tiny amounts in natural radioactive decay chains, but also produced by bombarding bismuth with energetic atomic particles.

Discovered in 1940, astatine (element number 85) leaves a “curious void” in the periodic table of the elements: Its properties in solid or liquid, or condensed form, are simply unknown, report researchers.

[related]

Don’t touch

In a new study, they theorize how astatine would look and behave were scientists able to make enough of it to touch and observe (although it would be extremely radioactive).

Condensed astatine, according to electronic property theories, would be monatomic—meaning the atoms do not form molecular pairs—and metallic, making it an unusual addition to the periodic table’s halogen group.

When other halogens are in condensed form under normal atmospheric pressures, “nothing much really happens to them,” says Cornell University physicist Neil Ashcroft, for they remain in pairs of atoms, as they do in the gas form.

But as astatine is condensed, they predict, the pairing comes apart and the element becomes monatomic and simultaneously metallic—a bit like mercury (another chemical curiosity, notes chemist Roald Hoffmann), but not in liquid form.

No squeezing required

Scientists have made other halogens metallic by “squeezing,” or applying pressure to their molecular structures. But in the new paper, the team reports that astatine requires no squeezing—it becomes metallic the moment it is condensed.

In the 70 years since its discovery, there has been little study of astatine, even though it has some medical applications. However, advances in supercomputing and in theories of electronic structure of condensed matter are allowing scientists to make predictions about the fleeting element.

They can then study the even more exotic “superheavy” elements further down the periodic table, part of a promised “island of stability” of long-lived, non-decaying elements, Ashcroft notes.

The largest amount of astatine ever created is 0.05 micrograms—not very much at all, Hoffmann says, though that it is still around 140,000 billion atoms. He, Ashcroft, and others plan to keep exploring some of the fundamental mysteries in the Periodic Table.

“It’s just sheer fun,” Hoffmann says.

Ashcroft, professor of physics emeritus; Hoffmann, professor in humane letters emeritus; and former postdoctoral associate Andreas Hermann, now at the University of Edinburgh, are co-authors of the study, which is accepted for publication in Physical Review Letters.

The National Science Foundation and the US Department of Energy supported the research.

Source: Cornell University