Astatine (At, atomic number 85) is a chemical element that exists in only trace amounts on Earth. It is the rarest naturally occurring element in the planet’s crust, generated solely as a transient decay product of heavier elements. Because of this extreme scarcity, a bulk, visible sample of astatine has never been successfully assembled for direct measurement. Consequently, many of its fundamental physical properties, including its boiling point, are theoretical values derived from scientific modeling rather than direct observation.
The Highly Estimated Boiling Point
The boiling point of astatine is not a measured quantity but an estimated value based on predictable trends within the halogen group. Scientists predict the boiling point falls within a range, with one widely cited value being approximately \(337^\circ\text{C}\). This prediction is calculated using extrapolation, extending the observed increase in boiling points from the lighter halogens: fluorine, chlorine, bromine, and iodine.
The increasing atomic size and mass down the group lead to stronger intermolecular forces, which raise the boiling point. While \(337^\circ\text{C}\) is a common theoretical figure, other models suggest a potentially lower value, with some estimates falling closer to \(230^\circ\text{C}\). This uncertainty highlights the challenge of predicting the behavior of the heaviest elements, where relativistic effects can alter expected chemical properties.
Factors Complicating Astatine Measurement
The primary reason the boiling point is an estimate is the element’s intense radioactivity. All of astatine’s isotopes are highly unstable; the most stable one, Astatine-210, has a half-life of only \(8.1\) hours. This short half-life means that any atoms created decay almost immediately into other elements.
The rapid decay process releases a significant amount of heat energy, posing a substantial hurdle to measurement. A macroscopic, weighable sample of astatine would generate so much heat from its own radioactive decay that it would instantly vaporize itself. Experiments must instead rely on studying trace amounts using techniques like radio gas chromatography to infer its thermodynamic behavior.
General Characteristics of Element 85
Astatine is positioned in Group 17 of the periodic table, making it the heaviest member of the halogen family, below iodine. Unlike its lighter counterparts, which include gases and a liquid, astatine is predicted to be a solid at standard temperature and pressure. It is also expected to exhibit metallic or metalloid characteristics, diverging from the nonmetallic nature of the other halogens.
Due to its short life and high energy decay, Astatine-211 is of particular interest in scientific research and nuclear medicine. Its use is explored in targeted alpha therapy, where the isotope is attached to biological molecules to deliver radiation directly to cancerous cells.