Astatine

Astatine is a radioactive chemical element with the chemical symbol At and atomic number 85. It occurs on Earth as the result of the radioactive decay of certain heavier elements. All of its isotopes are short-lived; the most stable is astatine-210, with a half-life of 8.1 hours. Accordingly, much less is known about astatine than most other elements. The observed properties are consistent with it behaving as a heavier analog of iodine; many other properties have been estimated based on this resemblance.

Elemental astatine has never been viewed, because a mass large enough to be seen by the naked eye would be immediately vaporized by the heat generated by its own radioactivity. Astatine may have a dark or metallic appearance and be a semiconductor, or it may be a metal. It is likely to have a higher melting point than iodine, on a par with those of bismuth and polonium. Chemically, astatine can behave as a halogen (the group of elements including chlorine and fluorine), and could be expected to form ionic astatides with alkali or alkaline earth metals; it is known to form covalent compounds with nonmetals, including other halogens. It can also behave as a metal, with a cationic chemistry that distinguishes it from the lighter halogens. The second longest-lived isotope of astatine, astatine-211, is the only one with any commercial application, being used in medicine to diagnose and treat some diseases via its emission of alpha particles. Only extremely small quantities are used due to its intense radioactivity.

The element was first produced by Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè at the University of California, Berkeley in 1940. They named it "astatine", after the Greek astatos (αστατος) meaning "unstable". Three years later it was found in nature, although it is the least abundant of the non-transuranic elements in the Earth's crust, with much less than one gram being present at any given time. Six astatine isotopes, with mass numbers of 214 to 219, occur naturally as the decay products of various heavier elements, but neither the most stable isotope astatine-210 nor the medically useful astatine-211 occurs naturally.

§Characteristics

Astatine is an extremely radioactive element; all its isotopes have half-lives of less than 12 hours, decaying into bismuth, polonium, radon, or other astatine isotopes. Of the first 101 elements in the periodic table, only francium is less stable.

The bulk properties of astatine are not known with any certainty. Research is limited by its short half-life, which prevents the creation of weighable quantities. A visible piece of astatine would be immediately vaporized due to the heat generated by its intense radioactivity. Astatine is usually classified as either a nonmetal or a metalloid. Metal formation for astatine has also been predicted.

§Physical

Most of the physical properties of astatine have been estimated (by interpolation or extrapolation), using theoretically or empirically derived methods. For example, heavier halogens are darker than halogens of lesser atomic weight â€" fluorine is nearly colorless, chlorine is green, bromine is brown, and iodine is dark gray/violet. Astatine is sometimes described as being a black solid (assuming that it follows this trend), or as having a metallic appearance (if it is a metalloid or a metal). The melting and boiling points of astatine are also expected to follow the trend seen in the halogen series, increasing with atomic number. On this basis, the melting and boiling points are estimated to be 575 and 610 K (302 and 337 °C; 575 and 638 °F), respectively. Some experimental evidence suggests astatine may have lower melting and boiling points than those implied by the halogen trend. Astatine sublimes less readily than does iodine, having a lower vapor pressure. Even so, half of a given quantity of astatine will vaporize in an hour if put on a clean glass surface at room temperature.

The structure of solid astatine is unknown. Evidence for (or against) the existence of diatomic astatine (At₂) is sparse and inconclusive. Some sources state that At₂ does not exist, or at least has never been observed, while other sources assert or imply its existence. Despite this controversy, many properties of diatomic astatine have been predicted; for example, its density would be 6.2â€"6.5 g/cm³.

§Chemical

Many chemical properties of astatine have been observed using tracer studies on extremely dilute astatine solutions, typically less than 10^âˆ'10 mol/L. Some properties â€" such as anion formation â€" are in line with other halogens. Astatine has some metallic characteristics as well, such as plating onto a cathode, coprecipitating with metal sulfides in hydrochloric acid, and forming a cation in strong acidic solutions, and complexes with EDTA, a chelating agent. The chemistry of astatine is "clouded by the extremely low concentrations at which astatine experiments have been conducted, and the possibility of reactions with impurities, walls and filters, or radioactivity by-products, and other unwanted nano-scale interactions."

Astatine has an electronegativity of 2.2 on the revised Pauling scale. This is lower than that of iodine (2.66) and the same as hydrogen. In hydrogen astatide (HAt) the negative charge is predicted to be on the hydrogen atom, implying that this compound should instead be referred to as astatine hydride. That would be consistent with the electronegativity of astatine on the Allredâ€"Rochow scale (1.9) being less than that of hydrogen (2.2). The electron affinity of astatine is predicted to be reduced by one-third due to spin-orbit interactions.

§Compounds

Astatine is the least reactive of the halogens, being less reactive than iodine, but multiple compounds of astatine have been synthesized in microscopic amounts and studied as intensively as possible before their radioactive disintegration. The reactions involved are normally tested with dilute solutions of astatine mixed with larger amounts of iodine. The iodine acts as a carrier, ensuring that there is sufficient material for laboratory techniques (such as filtration and precipitation) to work.

Only a few compounds with metals have been reported, including those of sodium, caesium, palladium, silver, and lead. Some characteristic properties of silver astatide, and the known and hypothetical alkali and alkaline earth astatides, have been estimated by extrapolation from other silver or alkali or alkaline earth halides. Caesium diiodoastatate(I) CsAtI₂ has been prepared.

The formation of an astatine compound with hydrogen â€" usually referred to as hydrogen astatide â€" was noted by the pioneers of astatine chemistry. As mentioned, there are grounds for referring to this compound as astatine hydride instead. It is easily oxidized; acidification by (dilute) nitric acid gives the At⁰ or At⁺ forms, and the addition of silver(I) then precipitates astatine, only partially as silver(I) astatide (AgAt) (or not at all). Iodine, in contrast, is not oxidized, and precipitates readily as silver(I) iodide.

Astatine is known to bind to boron, carbon, and nitrogen. Various boron cage compounds have been prepared with Atâ€"B bonds, these being more stable than Atâ€"C bonds. Carbon tetraastatide (CAt₄) has been synthesized. Astatine can replace a hydrogen atom in benzene to form astatobenzene C₆H₅At; this may be oxidized to C₆H₅AtCl₂ by chlorine. By treating this compound with an alkaline solution of hypochlorite, C₆H₅AtO₂ can be produced. In the molecules dipyridine-astatine(I) perchlorate [At(C₅H₅N)₂][ClO₄] and the analogous nitrate, the astatine atom is bonded to each nitrogen atom in the two pyridine rings.

With oxygen, there is evidence of the species AtO^âˆ', AtOâˆ'
2, and AtO⁺ in aqueous solution, formed by the reaction of astatine with an oxidant such as elemental bromine or (in the last case) by sodium persulfate in a solution of perchloric acid. The well characterized AtOâˆ'
3 anion can be obtained by, for example, the oxidation of astatine with potassium hypochlorite in a solution of potassium hydroxide. Further oxidation, such as by xenon difluoride (in a hot alkaline solution) or periodate (in a neutral or alkaline solution), yields the perastatate ion AtOâˆ'
4; this is only stable in neutral or alkaline solutions. Astatine is also thought to be capable of forming cationic salts with oxyanions such as iodate or dichromate; this is based on the observation that, in acidic solutions, monovalent or intermediate positive states of astatine coprecipitate with the insoluble salts of metal cations such as silver(I) iodate or thallium(I) dichromate.

Astatine may form bonds to the other chalcogens; these include S₇At⁺ and At(CSN)âˆ'
2 with sulfur, a coordination selenourea compound with selenium, and an astatineâ€"tellurium colloid with tellurium.

Astatine is known to react with its lighter homologs iodine, bromine, and chlorine in the vapor state; these reactions produce diatomic interhalogen compounds with formulas AtI, AtBr, and AtCl. The first two compounds may also be produced in water â€" astatine reacts with iodine/iodide solution to form AtI, whereas AtBr requires (aside from astatine) an iodine/iodine monobromide/bromide solution. The excess of iodides or bromides may lead to AtBrâˆ'
2 and AtIâˆ'
2 ions, or in a chloride solution, they may produce species like AtClâˆ'
2 or AtBrClâˆ'
via equilibrium reactions with the chlorides. Oxidation of the element with dichromate (in nitric acid solution) showed that adding chloride turned the astatine into a molecule likely to be either AtCl or AtOCl. Similarly, AtOClâˆ'
2 or AtClâˆ'
2 may be produced. In a plasma ion source mass spectrometer, the ions [AtI]⁺, [AtBr]⁺, and [AtCl]⁺ have been formed by introducing lighter halogen vapors into a helium-filled cell containing astatine, supporting the existence of stable neutral molecules in the plasma ion state. No astatine fluorides have been discovered yet. Their absence has been speculatively attributed to the extreme reactivity of such compounds, including the reaction of an initially formed fluoride with the walls of the glass container to form a non-volatile product. Thus, although the synthesis of an astatine fluoride is thought to be possible, it may require a liquid halogen fluoride solvent, as has already been used for the characterization of radon fluoride.

§History

In 1869, when Dmitri Mendeleev published his periodic table, the space under iodine was empty; after Niels Bohr established the physical basis of the classification of chemical elements, it was suggested that the fifth halogen belonged there. Before its officially recognized discovery, it was called "eka-iodine" (from Sanskrit eka â€" "one") to imply it was one space under iodine (in the same manner as eka-silicon, eka-boron, and others). Scientists tried to find it in nature; given its rarity, these attempts resulted in several false discoveries.

The first claimed discovery of eka-iodine was made by Fred Allison and his associates at the Alabama Polytechnic Institute (now Auburn University) in 1931. The discoverers named element 85 "alabamine", and assigned it the symbol Ab, designations that were used for a few years. In 1934 H. G. MacPherson of University of California, Berkeley disproved Allison's method and the validity of his discovery. There was another claim in 1937, by the chemist Rajendralal De. Working in Dacca in British India (now Dhaka in Bangladesh), he chose the name "dakin" for element 85, which he claimed to have isolated as the thorium series equivalent of radium F (polonium-210) in the radium series. The properties he reported for dakin do not correspond to those of astatine, and the true identity of dakin is not known.

In 1940, the Swiss chemist Walter Minder announced the discovery of element 85 as the beta decay product of radium A (polonium-218), choosing the name "helvetium" (from Helvetia, "Switzerland"). Berta Karlik and Traude Bernert were unsuccessful in reproducing his experiments, and subsequently attributed Minder's results to contamination of his radon stream (radon-222 is the parent isotope of polonium-218). In 1942, Minder, in collaboration with the English scientist Alice Leigh-Smith, announced the discovery of another isotope of element 85, presumed to be the product of thorium A (polonium-216) beta decay. They named this substance "anglo-helvetium", but Karlik and Bernert were again unable to reproduce these results.

Later in 1940, Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè isolated the element at the University of California, Berkeley. Instead of searching for the element in nature, the scientists created it by bombarding bismuth-209 with alpha particles in a cyclotron (particle accelerator) to produce, after emission of two neutrons, astatine-211. The name "astatine" comes from the Greek astatos (αστατος) meaning "unstable", due to its propensity for radioactive decay, with the ending "-ine", found in the names of the four previously discovered halogens. Three years later, astatine was found as a product of naturally occurring decay chains by Karlik and Bernert. Since then, astatine has been determined to be in three out of the four natural decay chains.

Corson and his colleagues classified astatine as a metal on the basis of its analytical chemistry. Subsequent investigators reported iodine-like, cationic, or amphoteric behaviour. In a 2003 retrospective, Corson wrote that "some of the properties [of astatine] are similar to iodine…it also exhibits metallic properties, more like its metallic neighbors Po and Bi."

§Isotopes

There are 32 known isotopes of astatine, with atomic masses (mass numbers) of 191 and 193â€"223. No stable or long-lived astatine isotope is known, and no such isotope is expected to exist.

Astatine has 23 nuclear isomers, which are nuclei with one or more nucleons (protons or neutrons) in an excited state. A nuclear isomer may also be called a "meta-state", meaning the system has more internal energy than the "ground state" (the state with the lowest possible internal energy), making the former likely to decay into the latter. There may be more than one isomer for each isotope. The most stable of these nuclear isomers is astatine-202m1, which has a half-life of about 3 minutes, longer than those of all the ground states except for those of isotopes 203â€"211 and 220. The least stable is astatine-214m1; its half-life of 265 nanoseconds is shorter than those of all ground states except that of astatine-213.

Astatine's alpha decay energies follow the same trend as for other heavy elements. Lighter astatine isotopes have quite high energies of alpha decay, which become lower as the nuclei become heavier. Astatine-211 has a significantly higher energy than the previous isotope, because it has a nucleus with 126 neutrons, and 126 is a magic number corresponding to a filled neutron shell. Despite having a similar half-life to the previous isotope (8.1 hours for astatine-210 and 7.2 hours for astatine-211), the alpha decay probability is much higher for the latter: 41.81% against only 0.18%. The two following isotopes release even more energy, with astatine-213 releasing the most energy of all astatine isotopes. For this reason, it is the shortest-lived astatine isotope. Even though heavier astatine isotopes release less energy, no long-lived astatine isotope exists, due to the increasing role of beta decay (electron emission). This decay mode is especially important for astatine; as early as 1950 it was postulated that all isotopes of the element undergo beta decay. Beta decay modes have been found for all astatine isotopes except astatine-213, -214, -215, and -216m. Astatine-210 and lighter isotopes exhibit beta plus decay (positron emission), astatine-216 and heavier isotopes exhibit beta (minus) decay, and astatine-212 decays via both modes, while astatine-211 undergoes electron capture.

The most stable isotope is astatine-210, which has a half-life of 8.1 hours. The primary decay mode is beta plus, to the relatively long-lived (in comparison to astatine isotopes) alpha emitter polonium-210. In total, only five isotopes have half-lives exceeding one hour (astatine-207 to -211). The least stable ground state isotope is astatine-213, with a half-life of 125 nanoseconds. It undergoes alpha decay to the extremely long-lived bismuth-209.

§Natural occurrence

Astatine is the rarest naturally occurring element that is not a transuranic element, with the total amount in the Earth's crust estimated to be less than one gram at any given time. Any astatine that was present at the Earth's formation has long since decayed, and extant astatine has formed through the decay of heavier elements. It was previously thought to be the rarest element occurring on the Earth, but has lost this status to berkelium, atoms of which can be produced by neutron capture reactions and beta decay in very highly concentrated uranium-bearing deposits.

Six astatine isotopes occur naturally (astatine-214 to -219). Because of their short half-lives, they are found only in trace amounts. There are no data indicating that astatine occurs in stars.

Four of these isotopes (astatine-215, -217, -218, and -219) are found due to their production in major natural decay chains. Francium-223, the father isotope of astatine-219, alpha decays with a probability of only 0.006%, making this astatine isotope extremely rare compared to other astatine isotopes; this is in spite of its half-life of 56 seconds being the longest of the natural astatine isotopes. Astatine-219 decays to polonium-215, which beta decays, with a smaller probability of 0.00023%, to astatine-215. The landmass of North and South America combined, to a depth of 16 kilometers (10 miles), contains only about one trillion astatine-215 atoms at any given time (about 3.5 × 10^âˆ'10 grams). Astatine-218 is found in nature as a result of polonium-218 beta decay; as with francium-223 and polonium-215, decay to an astatine isotope is not the primary decay mode. The astatine-217 isotope has a straight chain leading to astatine; its father isotope (francium-221) decays exclusively to this nuclide. As its fathers, grandfathers, and so on each decay exclusively to one nuclide, this gives only one possible way for the starting nuclide in the neptunium series (neptunium-237) to decay â€" via eventual production of astatine-217.

Astatine-214, -215, and -216 result from the triple alpha decay of naturally occurring isotopes protactinium-226, -227, and -228. One or more of these lighter astatine isotopes (as well as At-217 to -219) are sometimes not listed as naturally occurring due to misconceptions that astatine has no naturally occurring isotopes, or discrepanices in the literature.

§Synthesis

§Formation

Astatine was first produced by bombarding bismuth-209 with energetic alpha particles, and this is still the major route used to create the relatively long-lived isotopes astatine-209 through astatine-211. Astatine is only produced in microscopic quantities, with modern techniques allowing production runs of up to 6.6 gigabecquerels (about 86 nanograms or 2.47 × 10¹⁴ atoms).

The most important isotope is astatine-211, the only one in commercial use. To produce the bismuth target, the metal is sputtered onto a gold, copper, or aluminium surface at 50 to 100 milligrams per square centimeter. The bismuth layer, or alternatively bismuth oxide, is forcibly fused with a copper plate. The target is kept under a chemically neutral nitrogen atmosphere, and is cooled with water to prevent premature astatine vaporization. In a particle accelerator, such as a cyclotron, alpha particles are collided with the bismuth. Even though only one bismuth isotope is used (bismuth-209), the reaction may occur in three possible ways, producing astatine-209, astatine-210, or astatine-211. In order to eliminate undesired nuclides, the maximum energy of the particle accelerator is set to a value (about 28.4 MeV) above that for the reaction producing astatine-211 (to produce the desired isotope) and below the one producing astatine-210 (to avoid producing other astatine isotopes).

§Separation

Since the element is the main product of the synthesis, after its formation it must only be separated from the target and traces of other radioisotopes. The astatine-containing target is heated to 270 °C (518 °F) to vaporize volatile radioisotopes, after which the temperature is raised to 800 °C (1,470 °F). Astatine's vaporization does not occur at an adequate rate at temperatures below 600 °C (1,112 °F), but at temperatures above 800 °C (1,470 °F), its volatility from a bismuth surface increases significantly. The condensed vapor (distillate) is collected on a water-cooled platinum surface, which is later moved into a U-like quartz vessel. The quartz vessel is heated to 130 °C (266 °F) to remove further traces of impurities (typically polonium) and then to 500 °C (932 °F) to remove astatine, which is collected on a cold finger. The purified element is then washed off the cold finger with a dilute nitric acid solution. Using this technique, yields of astatine of up to 30% may be achieved.

§Chemical routes

Astatine may be extracted from acidic aqueous solutions using organic solvents. The distribution coefficient varies with the solvent: the highest is 200, for a 0.01 M nitric acid/benzene system. Like iodine, it dissolves in benzene, carbon disulfide, and tetrachloromethane (therefore, these have high distribution coefficients in systems with water), but, unlike iodine, it cannot be extracted from alkaline solutions of organic solvents, as it reacts with bases and disproportionates. Diatomic interhalogen molecules (with iodine and bromine) have smaller distribution coefficients than dihalogens (where both atoms are the same), as the former are dipoles. Separation from other elements is done via extraction from hydrochloric acid/isopropyl ether solutions. Iron(III) hydroxide may be used to remove further traces. Using 8 M HCl solutions, yields may be as high as 90%.

When astatine is introduced into negatively charged chloride complexes and dissolved in chloride or hydrogen chloride (5â€"8 M at best, because the complex decomposes on a lower chloride concentration), such a complex can be absorbed on a cation exchanger; the same holds for the astatine cation. The astatine is originally inserted into tellurium, which (together with any polonium impurity) is washed away with hydrochloric acid/chlorine solution. Astatine is later extracted via chlorine water. The cation resin is processed in a nitric acid solution (with a small dichromate content), and then with a nitric acid solution (to remove the chloride ions). Astatine is then extracted with nitric acid/dichromate solution.

Astatine partially coprecipitates from weakly acidic solutions with several hydroxides, silver(I) and thallium(I) iodides, several sulfides of heavy metals, silver metal, and tellurium. This is likely to be caused by adsorption on the surface of the precipitate. The adsorption is suppressed by increasing the acidity of the solution, washing off the precipitate with acetone, or adding iodine; the latter proves the adsorption character of the astatine coprecipitation. The most important reaction is that with tellurium, which is catalyzed by reducing agents like tin dichloride; the reaction does not occur in alkaline solutions. The quantity of astatine precipitated does not rely on the tellurium amount, and reaches 90% in concentrated HCl (this reaction also eliminates any impurities). Astatide and astatate have been shown to coprecipitate with iodide and iodate; they cannot be washed off with acetone. These methods are typically used when astatine is the result of a different, rarer reaction of bismuth, lead, or thorium with high-energy protons.

§Uses and precautions

Newly formed astatine-211 is important in nuclear medicine. It must be used quickly as it decays with a half-life of 7.2 hours; this is long enough to permit multi-step labeling strategies. Astatine-211 can be used for targeted alpha particle radiotherapy, since it decays either via emission of an alpha particle (to bismuth-207), or via electron capture (to an extremely short-lived nuclide, polonium-211, which undergoes further alpha decay).

Similarly to iodine, although to a lesser extent, astatine is preferentially concentrated in the thyroid gland. If administered in the form of a radiocolloid it tends to concentrate in the liver. The principal medicinal difference between astatine-211 and iodine-131 (a radioactive iodine isotope also used in medicine) is that iodine-131 emits high energy beta particles, and astatine does not. Beta particles have much greater penetrating power through tissues than do the much heavier alpha particles. An average alpha particle released by astatine-211 can travel up to 70 µm through the surrounding tissues; an average energy beta particle emitted by iodine-131 can travel nearly 30 times as far, to about 2 mm. Thus, using astatine-211 instead of iodine-131 enables the thyroid to be dosed appropriately, while the neighboring parathyroid gland is spared. The short half-life and limited penetrating power of its radiation through tissues renders astatine generally preferable to iodine-131 when used in diagnosis.

Experiments in rats and monkeys suggest that astatine-211 causes much greater damage to the thyroid gland than does iodine-131, with repetitive injection of the nuclide resulting in necrosis and cell dysplasia within the gland. These experiments also suggest that astatine could cause damage to the thyroid of any organism. Early research suggested that injection of lethal quantities of astatine caused morphological changes in breast tissue (although not other tissues); this conclusion remains controversial.

§See also

• Ununseptium
• Radiation protection

§Notes

§References

§Bibliography

• Corson, D. R.; MacKenzie, K. R.; Segrè, E. (1940). "Artificially Radioactive Element 85". Physical Review 58 (8): 672â€"678. doi:10.1103/PhysRev.58.672.  (subscription required)
• Kugler, H. K.; Keller, C. (1985). 'At, Astatine', System No. 8a. Gmelin Handbook of Inorganic and Organometallic Chemistry 8 (8th ed.). Springer-Verlag. ISBN 3-540-93516-9. 
• Lavrukhina, A. K.; Pozdnyakov, A. A. (1966). Аналитическая химия технеция, прометия, астатина и франция [Analytical Chemistry of Technetium, Promethium, Astatine, and Francium] (in Russian). Nauka. 
• Zuckerman, J. J.; Hagen, A. P. (1989). Inorganic Reactions and Methods, the Formation of Bonds to Halogens. John Wiley & Sons. ISBN 978-0-471-18656-4. 

§External links

• Astatine at The Periodic Table of Videos (University of Nottingham)
• Astatine: Halogen or Metal?

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