Astatine has not been shown to form a directly characterized bulk metal. Instead, a 2013 theoretical study predicted that condensed astatine could be a monatomic metal at 1 atm and might be superconducting. That prediction concerns a hypothetical condensed phase, not an observed sample.
What metallic properties were predicted for astatine?
In a 2013 study, Andreas Hermann, Roald Hoffmann, and N. W. Ashcroft used density-functional theory and structural-selection methods to examine condensed astatine, including relativistic physics important for such a heavy element. They predicted that at 1 atm it would be metallic and monatomic, and that it might be superconducting. The paper’s abstract describes a calculation, not an experimental observation of bulk astatine. Read the study’s PubMed record.
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The prediction is striking because astatine sits below iodine in the halogen group, yet the study suggests its condensed elemental form could behave unlike iodine in important ways. It does not establish that astatine is metallic under every pressure or temperature, nor does the accessible abstract specify a crystal lattice.
Why is the prediction difficult to test?
Astatine is dangerously radioactive, and only tiny quantities have ever been produced—not weighable amounts, according to the Royal Society of Chemistry. Its most stable isotopes have half-lives on the order of hours: the RSC lists 8.1 hours for astatine-210 and 7.21 hours for astatine-211. These constraints make direct characterization of a bulk sample exceptionally difficult. Royal Society of Chemistry: Astatine.
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Does measured astatine chemistry confirm that it is a metal?
No. A 2020 study measured astatine’s atomic electron affinity as 2.41578(7) eV and reported a relativistic theoretical value of 2.414(16) eV, in agreement with the measurement. Electron affinity is an atomic property; it is not a measurement of electrical conductivity, superconductivity, or a bulk crystal structure. The 2020 study in Nature Communications.
Separately, the RSC reports that mass spectrometry confirmed astatine behaves chemically like other halogens, particularly iodine. That evidence concerns chemical behavior, while the 2013 calculation concerns a predicted condensed elemental phase. The findings address different questions, so they are not a straightforward contradiction.
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Is astatine a halogen, metalloid, or metal?
“Halogen” identifies astatine’s periodic-table family and reflects its chemical relationships. “Metal” in the 2013 paper describes predicted behavior of condensed elemental astatine. A chemistry education review notes that astatine may reasonably be classed as either a metalloid or a metal depending on which anticipated, observed, or predicted properties are emphasized. Journal of Chemical Education: Which Elements Are Metalloids?
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So the careful answer depends on what is being classified: astatine is a halogen by its position and chemical family, while metallic behavior in a condensed phase remains a theoretical prediction rather than a verified bulk property.
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