Astatine: The Rarest Element on Earth

Astatine is the rarest naturally occurring element in the Earth's crust, with an estimated total of less than 1 gram present at any time due to its short half-life. Its most stable isotope, astatine-210, has a half-life of approximately 8.1 hours, meaning it rapidly decays into other elements, such as polonium or bismuth, through alpha or beta decay. This instability makes astatine challenging to study, and its physical and chemical properties are inferred from limited experiments and theoretical models.

What is Astatine?

Astatine (symbol: At, atomic number: 85) is a rare, radioactive chemical element in the halogen group of the periodic table, positioned below iodine. Discovered in 1940 by Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè at the University of California, Berkeley, it was first synthesized by bombarding bismuth-209 with alpha particles. The name "astatine" derives from the Greek word astatos, meaning "unstable," reflecting its highly radioactive nature.

The Rarest Natural Element on Earth. This mineral, Autunite, appears in my Photographic Periodic Table Poster representing astatine, because this highly unstable element can't reasonably be photographed.

As a halogen, astatine shares characteristics with fluorine, chlorine, bromine, and iodine, but it exhibits unique traits due to its metallicity and relativistic effects from its heavy nucleus. It is expected to be a dark, lustrous solid at room temperature, though its appearance is not well-documented due to its scarcity and rapid decay.

Astatine likely forms diatomic molecules (At₂) and can bond with other elements, showing both nonmetallic and metallic behavior. Its chemistry is complex, with potential applications in nuclear medicine, particularly for targeted alpha therapy in cancer treatment, due to its alpha-particle emissions.

Challenges in Study:

Extreme Rarity: Not enough material to work with using traditional chemical methods.

Intense Radioactivity: The radiation it emits causes self-heating and radiolytic decomposition of itself and any compounds it forms, making bulk characterization nearly impossible.

Short Half-Life: Experiments must be designed and executed very quickly.

Most of what we know about Astatine's chemistry comes from tracer studies, where infinitesimally small quantities are used, or from theoretical calculations.

Astatine Properties:

Appearance: Predicted to be a dark, lustrous solid at room temperature, possibly resembling iodine but with metallic characteristics. Its appearance is not well-documented due to rapid decay.

State at Room Temperature: Likely solid, though it may sublime like iodine under certain conditions.

Melting Point: Estimated at ~300–302°C (573–575 K), based on trends in the halogen group.

Boiling Point: Estimated at ~337–350°C (610–623 K), though not precisely measured.

Density: Predicted to be around 6.2–6.5 g/cm³, inferred from its position in the periodic table.

Crystal Structure: Likely forms a diatomic structure (At₂) in its elemental form, with possible metallic bonding tendencies.

Nuclear Properties

Isotopes: Astatine has no stable isotopes. Over 30 isotopes are known, with astatine-210 (half-life ~8.1 hours) being the most stable. Astatine-211 (half-life ~7.2 hours) is notable for medical applications.

Radioactivity: Highly radioactive, decaying via alpha or beta emission. For example, astatine-210 decays into polonium-206 or bismuth-206.

Half-Life: Short half-lives (seconds to hours) make astatine difficult to study and limit its accumulation in nature.

Decay Products: Common decay products include isotopes of polonium, bismuth, or lead, depending on the isotope and decay mode.