Astatine

• 1. Introduction

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  Astatine, element 85 on the periodic table, has never been seen as a solid by any human being. The reason is startling. A visible speck of it would heat itself so fiercely through its own radioactivity that it would turn to vapor at once. Its symbol is At, and it sits in the column of the halogens, below fluorine, chlorine, bromine and iodine. Yet it behaves unlike the others in puzzling ways, and some of its traits look almost like those of a metal. How can chemists know anything about a substance that vanishes before it can be weighed? Why did at least five separate scientists announce its discovery before anyone got it right? And why do cancer researchers still pursue an element that exists in such tiny, fleeting amounts?

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• 2. The Rarest Element

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  Roughly one trillion atoms of astatine-215 exist at any moment in the landmass of North and South America, down to a depth of 16 kilometers. That is only about 3.5 times ten to the minus tenth of a gram. Across the whole crust of the Earth, some estimates put the total at less than one gram. Others allow for as much as one ounce, about 28 grams. Either way, astatine is the rarest element found in nature.

  None of it is left over from the formation of the planet. Four isotopes occur naturally: astatine-215, -217, -218 and -219. Each one is made again and again as thorium and uranium ores decay, and as trace neptunium-237 decays. Astatine-217 comes from neptunium-237, whose own half-life is 2.14 million years. That is too brief for any original supply to have survived, so the neptunium now exists only as a by-product of reactions inside uranium ores.

  Astatine-218 was the first of the natural isotopes to be identified. Astatine-219 lasts the longest of the four, with a half-life of just 56 seconds. A fifth candidate, astatine-216, has sometimes been listed as natural. The reports of seeing it were described as doubtful and have never been confirmed. Some reference lists leave natural astatine out altogether, because of lingering misconceptions or inconsistencies in the literature.

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• 3. Chemistry Without a Sample

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  The researchers who study astatine describe their own field as clouded. Their experiments run at concentrations typically below ten to the minus ten moles per liter. At that scale, a reaction with a filter wall, an impurity or a radioactive by-product can easily be mistaken for the behavior of astatine itself.

  So most of the physical picture is estimated. Halogens darken as they grow heavier: fluorine is nearly colorless, chlorine yellow-green, bromine red-brown and iodine dark gray or violet. Following that trend, astatine is often pictured as a black solid. If it is instead a metalloid or a metal, it could look metallic. On a clean glass surface at room temperature, half of a given amount would vaporize in about an hour.

  The crystal structure is unknown. Like iodine, it might form an orthorhombic arrangement of two-atom molecules and act as a semiconductor with a band gap of 0.7 electron volts. Under a metallic phase, it might take a face-centered cubic form and even become a superconductor. A metallic version is predicted to have a density of 8.91-8.95 grams per cubic centimeter. Even the existence of diatomic astatine remains disputed, with some sources denying it has ever been observed.

  On the chemical side, astatine's electronegativity is 2.2 on the revised Pauling scale. That is below iodine's 2.66 and equal to hydrogen's. In hydrogen astatide, the negative charge is therefore predicted to sit on the hydrogen. This is why some chemists argue the compound should be called astatine hydride. The electron affinity of 233 kilojoules per mole is 21 percent below iodine's, a sharper drop than the other halogens show, and it has been attributed to spin-orbit interactions.

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• 4. A Halogen With Metal Habits

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  Dilute solutions of radioactive astatine, mixed with far larger amounts of iodine, are how its compounds have been studied. The iodine acts as a carrier, so that ordinary techniques such as filtration and precipitation have enough material to work on. In these tests astatine forms negative ions like the other halogens and takes odd-numbered oxidation states from -1 to +7. It is the least reactive halogen, though the next one down, tennessine, has not yet been investigated.

  Then the metallic side shows up. Astatine plates onto a cathode, and it coprecipitates with metal sulfides in hydrochloric acid. It forms complexes with EDTA, a metal-grabbing agent, and can serve as a metal when labeling antibodies. In the +1 state it resembles silver. Yet most of its organic chemistry matches iodine, including astatobenzene, where astatine replaces a hydrogen atom on a benzene ring. Boron cage compounds with At-B bonds prove more stable than those with At-C bonds.

  Silver gives a clean test of the difference. Iodine precipitates readily as silver iodide. Astatine, once oxidized by dilute nitric acid, may be only partly precipitated as silver astatide.

  Only a handful of metal astatides have been reported, with sodium, palladium, silver, thallium and lead. Astatine does react with iodine, bromine and chlorine in the vapor state, producing AtI, AtBr and AtCl. No astatine fluoride has been discovered. One speculation blames extreme reactivity, including a reaction of any fluoride with the glass container walls. A liquid halogen fluoride solvent, like the one used to characterize radon fluoride, might finally allow one to be made.

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• 5. Five False Discoveries

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  The space beneath iodine in Mendeleev's 1869 table sat empty for decades. Before anyone found the element that fills it, it was called eka-iodine, using the Sanskrit word for one to mean one place below iodine. Its rarity made the search treacherous, and several announcements turned out to be wrong.

  In 1931, Fred Allison and his associates at the Alabama Polytechnic Institute, now Auburn University, claimed success and named the element alabamine, with the symbol Ab. In 1934, H. G. MacPherson of the University of California, Berkeley, disproved Allison's method. In 1937, the chemist Rajendralal De, working in Dacca in British India, proposed the name dakin. His reported properties do not match astatine, and its radioactivity would have prevented the quantities he described. Astatine does not appear in the thorium series at all, so the true identity of dakin remains unknown.

  The Romanian physicist Horia Hulubei and the French physicist Yvette Cauchois reported element 85 in 1936, from its X-ray emission lines. Hulubei later chose the name dor, probably from the Romanian word for longing, since World War II had begun five years before. In French it would have become dorine in English. In 1947, the Austrian chemist Friedrich Paneth effectively rejected the claim. Hulubei's samples did contain astatine-218, but his detection was too weak to identify it, and he could not run chemical tests. An earlier mistaken claim about francium, element 87, also made colleagues discount his work.

  In 1940, the Swiss chemist Walter Minder announced helvetium, named for Helvetia, the Latin name of Switzerland. Berta Karlik and Traude Bernert could not reproduce it and blamed contamination of his radon stream. In 1942, Minder and the English scientist Alice Leigh-Smith announced anglo-helvetium. Karlik and Bernert again failed to reproduce the results.

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• 6. Made in a Cyclotron

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  Also in 1940, three scientists at Berkeley finally did it: Dale R. Corson, Kenneth Ross MacKenzie and Emilio Segrè. They did not hunt for the element in nature. They built it, by firing alpha particles from a cyclotron at bismuth-209. After two neutrons were emitted, the result was astatine-211.

  The team did not name their find at once. An element created artificially in invisible quantities, and not yet seen in nature, was not regarded as fully valid. Chemists were also slow to treat radioactive isotopes as equal to stable ones. Then in 1943, Berta Karlik and Traude Bernert found astatine in two natural decay chains, first the uranium series and then the actinium series. It was later seen in a third, the neptunium series.

  Paris made a plea in 1946. Friedrich Paneth called for synthetic elements to be recognized at last, citing the recent confirmation of astatine's natural occurrence among his reasons. He proposed that discoverers of unnamed elements name them. In early 1947, Nature published the Berkeley suggestion: astatine, from the Ancient Greek ástatos, meaning unstable. The ending followed the other halogens, whose names also described a property of the element.

  Corson's own view of the chemistry shifted over the years. His group first classed astatine as a metal. In a 2003 retrospective, he wrote that it resembles iodine in some properties but also shows metallic ones, "more like its metallic neighbors Po and Bi."

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• 7. Isotopes at the Edge

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  Of the 41 known isotopes of astatine, with mass numbers of 188 and 190-229, none is stable. Theoretical models suggest about 37 more could exist. The longest-lived is astatine-210, with a half-life of 8.1 hours, and only five isotopes, astatine-207 to -211, last longer than an hour. At the other end, astatine-213 survives for just 125 nanoseconds. It decays by alpha emission into bismuth-209, which is extremely long-lived.

  The number 126 explains one oddity. Astatine-211 has a nucleus with 126 neutrons, a magic number that corresponds to a filled neutron shell. Its alpha decay releases noticeably more energy than the isotope before it. The two near neighbors, astatine-210 and -211, have similar half-lives, 8.1 and 7.2 hours. But the probability of alpha decay is 41.81 percent for -211 and only 0.18 percent for -210.

  Heavier isotopes release less energy, yet none lasts long, because beta decay takes over. As early as 1950, researchers postulated that every astatine isotope undergoes beta decay. Mass measurements now suggest that astatine-215 is beta-stable, since it has the lowest mass of any isobar with mass number 215. Astatine-210 and most lighter isotopes emit positrons. Astatine-217 and heavier ones, except astatine-218, emit electrons. Astatine-211 captures an electron instead.

  The element also has 24 known nuclear isomers, excited versions of its nuclei. The longest-lived, astatine-202m1, has a half-life of about 3 minutes. The shortest, astatine-213m1, lasts 110 nanoseconds.

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• 8. Making It and Using It

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  At 650 degrees Celsius, the cyclotron target gives up its astatine. This dry method heats the irradiated bismuth until the astatine volatilizes into a cold trap, where yields of up to around 80 percent can be reached. Pushing the temperature to around 850 degrees may raise yields but risks carrying over bismuth, which can interfere with later labeling. The alternative wet approach dissolves the target in concentrated nitric or perchloric acid, then extracts the astatine with an organic solvent. One reported nitric acid separation recovered 93 percent, which fell to 72 percent after purification.

  The beam energy is capped on purpose. Higher energies make more astatine-211 but also astatine-210, which decays to toxic polonium-210. Operators therefore hold the accelerator at or just above the threshold for astatine-210. Production is limited by the scarcity of suitable cyclotrons and by the risk of melting the target. Even then, modern runs yield only up to about 86 nanograms.

  Astatine-211 is the only isotope in commercial use. Its half-life of 7.2 hours is long enough for multistep labeling chemistry. It decays by alpha emission, or by electron capture into polonium-211, and either route quickly reaches stable lead-207. Polonium X-rays between 77 and 92 kiloelectronvolts let researchers track it inside animals and patients.

  The medical appeal lies in the alpha particle. An alpha particle from astatine-211 travels up to 70 micrometers through tissue. A beta particle from iodine-131 goes nearly 30 times farther, to about 2 millimeters. That short reach suits cases where few tumor cells sit close to essential healthy tissue. In cell culture, between one and ten astatine-211 atoms per cell caused significant damage to human cancer cells.

  Obstacles remain. World War II delayed the research by close to a decade, and monoclonal antibodies to carry the element to a tumor did not arrive until the 1970s. Astatine also tends to detach from its carrier molecules, especially at sp3 carbon sites. Free astatine concentrates dangerously in the thyroid, and also in the lungs and spleen. In rats and monkeys, repeated doses of astatine-211 caused necrosis and cell dysplasia in the thyroid, a worse effect than iodine-131. Trace amounts are safe in a well-aerated fume hood, and researchers say the supply needed to treat a staggering number of patients does not yet exist.

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