Astatine (At), with the atomic number 85, is the rarest naturally occurring element on Earth, with less than 30 grams estimated to be present in the planet’s crust at any moment. Its extreme scarcity and highly unstable nature meant its discovery could not follow the traditional path of isolation from natural ores. The element’s history is a unique blend of theoretical prediction, repeated false claims, and artificial nuclear synthesis. The definitive identification of this final halogen member required a shift from traditional methods to atom-smashing.
The Predicted Element
The quest for element 85 began with the framework of the periodic table developed by Dmitri Mendeleev in 1869. Mendeleev’s periodic law allowed him to leave deliberate gaps, predicting properties based on position relative to known neighbors. The space directly below iodine (atomic number 53) in Group 17, the halogens, required an element provisionally referred to as “eka-iodine.” This hypothetical element was predicted to be a heavier, more metallic halogen than iodine, continuing the trend observed down the group. This theoretical basis spurred decades of attempts to locate the missing element with atomic number 85.
Initial Attempts at Natural Isolation
Astatine proved extremely difficult to find in nature because its longest-lived isotope has a half-life of only 8.1 hours. Its natural existence is limited to minute, transient amounts found as a minor branching product in the decay chains of uranium and thorium. Consequently, attempts to isolate a detectable quantity from natural sources were unsuccessful.
Throughout the 1930s, several researchers mistakenly announced the discovery of element 85 from radioactive minerals. In 1931, Fred Allison claimed to have found it, naming it “Alabamine” (Ab). Other misidentifications included “Dor” and “Helvetium.” These false claims were later disproved, as the methods used could not reliably detect such a short-lived element.
Successful Creation Through Nuclear Synthesis
The definitive discovery of element 85 was achieved through an artificial nuclear process in 1940. This breakthrough occurred at the University of California, Berkeley, by physicists Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè. The team utilized the cyclotron, a particle accelerator, to create the element.
The scientists bombarded a target of Bismuth-209 (\(^{209}\text{Bi}\)) with high-energy alpha particles (\(^4\text{He}\)). The alpha particles fused with the bismuth nuclei in an alpha-particle-induced reaction. This reaction synthesized the element: Bismuth-209 plus one alpha particle yielded Astatine-211 (\(^{211}\text{At}\)) and released two neutrons.
This experiment provided the first unambiguous proof of element 85’s existence, confirmed by its distinct radioactive decay characteristics. The use of a particle accelerator demonstrated that some elements could only be created artificially, allowing for the isolation of sufficient \(^{211}\text{At}\) for study.
Final Confirmation and Nomenclature
Following the successful synthesis, the researchers undertook chemical analysis to confirm the element’s identity. They found that the element displayed chemical behavior analogous to the other halogens, particularly iodine, but with the slightly more metallic character predicted for a heavier element. This validated its placement as the fifth member of the halogen family.
The discoverers officially proposed the name “Astatine” in 1947, assigning it the symbol At. The name originates from the Greek word astatos, meaning “unstable” or “unsteady.” This directly references the element’s highly radioactive nature and short half-life, the property that necessitated its artificial creation.