Astatine

polonium ← astatine → radon I ↑ At ↓ Uus 85At Periodic table
Appearance
black solid
General properties
Name, symbol, number	astatine, At, 85
Pronunciation	/ˈæstətiːn/ AS-tə-teen or /ˈæstətɨn/ AS-tət-in
Element category	halogens
Group, period, block	17, 6, p
Standard atomic weight	(210)
Electron configuration	[Xe] 4f14 5d10 6s2 6p5
Electrons per shell	2, 8, 18, 32, 18, 7 (Image)
Physical properties
Phase	solid
Melting point	575 K, 302 °C, 576 °F
Boiling point	610 K, 337 °C, 639 °F
Heat of vaporization	40 kJ·mol−1
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 361 392 429 475 531 607
Atomic properties
Oxidation states	-1, +1, +3, +5
Electronegativity	2.2 (Pauling scale)
Ionization energies	1st: 890±40 kJ·mol−1
Covalent radius	150 pm
Van der Waals radius	202 pm
Miscellanea
Magnetic ordering	no data
Thermal conductivity	1.7 W·m−1·K−1
CAS registry number	7440-68-8
Most stable isotopes
Main article: Isotopes of astatine
iso NA half-life DM DE (MeV) DP 209At syn 5.41 h β+ 3.486 209Po α 5.758 205Bi 210At syn 8.1 h β+ 3.981 210Po α 5.632 206Bi 211At syn 7.21 h ε 0.786 211Po α 5.983 207Bi

Astatine (/ˈæstətiːn/ AS-tə-teen or /ˈæstətɪn/ AS-tə-tin) is a radioactive chemical element with the symbol At and atomic number 85. It occurs on the Earth only as the result of the decay of heavier elements and decays away so rapidly that much less is known about this element than about its upper neighbors in the periodic table. Earlier studies have shown this element follows trends in the periodic table, being the heaviest known halogen, with melting and boiling points being higher than those of lighter halogens.

Until recently, most of the chemical characteristics of astatine were inferred from comparison with other elements; however, important experimental studies have been done. The main difference between astatine and iodine is that the HAt molecule is chemically a hydride rather than a halide; however, in a fashion similar to the lighter halogens, it can form ionic astatides with metals and has some properties similar to true hydrogen halides. Bonds to nonmetals result in positive oxidation states, with +1 best exemplified by monohalides and their derivatives, while the higher states are characterized by bonds to oxygen and carbon. Astatine iodide and several other compounds with other halogens have been successfully synthesized, although astatine fluoride has not. The second longest-lived isotope (²¹¹At) is the only one to find a commercial use, being useful as an alpha emitter in medicine; however, only extremely small quantities are used – in larger ones it is very hazardous, given its intense radioactivity.

Astatine was first produced by Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè at the University of California, Berkeley in 1940. Three years later, it was found in nature; however, with an estimated amount of less than 28 grams (0.99 oz) at any given time, astatine is the least abundant element in Earth's crust among non-transuranium elements. Among astatine isotopes, six (with mass numbers 214 to 219) are present in nature as the result of decay of heavier elements; however, the most stable isotope (²¹⁰At) and the industrially used ²¹¹At are not.

General characteristics

Astatine is a highly radioactive element: its isotopes have half-lives of under 12 hours, decaying into isotopes of bismuth, polonium, or radon, or into other astatine isotopes. Among the first 103 elements, only francium and nobelium – the latter of which does not occur in nature – are less stable than astatine. Its extremely short half-life allows production only in microscopic quantities, which limits research into astatine.

Astatine is often cited as either a nonmetal or a metalloid. Its most common properties are normal for a heavier (more metallic than iodine) halogen: Like other halogens, it is composed of diatomic At₂ molecules under standard conditions. Its melting and boiling point follow the trend in the halogen series, increasing with the increase in atomic number, and are measured to be 302 °C (575 K; 576 °F) and 337 °C (610 K; 639 °F), respectively. Like iodine, it is very volatile: at room temperature, half of a piece of astatine vaporizes in an hour if put on clean glass. Most chemical properties, such as forming an anion more easily than a cation, are in line with other halogens as well. However, it has a few metallic properties: it plates out on a cathode, forms a cation in strong acidic solutions, and so on.

The element is often cited to have an electronegativity of 2.2 (Pauling scale), since this is stated in Pauling's work. This is lower than that of iodine (2.5 in the original work and 2.66 now) and the same as hydrogen's; however, experiments have shown that the actual astatine electronegativity is slightly below that of hydrogen (see below). It sublimes more readily than iodine, with a lower vapor pressure; it also dissolves in water better than iodine.

History

Mendeleev's table of 1871, with an empty space at the "eka-iodine" position

In 1869, 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, "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, this led to 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, however, H. G. MacPherson of University of California, Berkeley disproved Allison's method and the discovery's validity. This erroneous discovery was followed by another claim in 1937, by the chemist Rajendralal De. Working in Dhaka, British India (now 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 its identity 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"). However, Berta Karlik and Traude Bernert were unsuccessful in reproducing his experiments and 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.

Emilio Segrè, one of the discoverers of astatine

In 1940, Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè finally 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. The name "astatine" comes from the Greek word αστατος astatos, meaning "unstable", due to the created isotope's propensity for radioactive decay – later, all isotopes of the element were shown to be unstable – and 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.

Chemical reactivity and compounds

Astatine is the least reactive of the halogens, being less reactive than iodine; however, multiple compounds of astatine have been synthesized in microscopic amounts and studied as intensively as possible before their inevitable radioactive disintegration. The reactions 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.

The most common compound of the element is hydrogen astatide. The hydrogen astatide molecule has been calculated to have a dipole moment of 0.06 debyes, with hydrogen carrying the partial negative charge. Because astatine has a lower electronegativity than hydrogen (unlike the other halogens), the molecule should more properly be called astatine monohydride; this reversal of polarity partially explains its lower stability compared to the hydrogen halides. Since it is easily oxidized, it is precipitated in aqueous nitric acid/silver(I) solution, forming silver(I) astatide, AgAt.

Astatine is known to react with its lighter homologues iodine, bromine, and chlorine in the vapor state; this reaction produces diatomic interhalogen compounds, with formulas AtI, AtBr, and AtCl. The first two compounds may also be produced in water: 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 AtBr−
2 and AtI−
2 ions; in a chloride solution, they may turn to species like AtCl−
2 or AtBrCl⁻ via equilibrium. No report of gas phase AtCl preparation has been shown, but oxidation of the element with dichromate (in nitric acid solution) showed that adding chloride turned the astatine into a molecule, either AtCl or AtOCl; similarly, AtOCl−
2 or AtCl−
2 may be produced. In a plasma ion source mass spectrometer, similar ions [AtI]⁺, [AtBr]⁺, and [AtCl]⁺ have been formed by introducing vapors of the lighter halogens to the helium-filled cell where astatine is situated, supporting the existence of stable neutral molecules in the plasma ion state. No astatine fluoride has been discovered yet, and although its synthesis is thought to be possible, it may require a liquid halogen fluoride solvent; this has already been used for the characterization of radon fluorides.

The lower oxidation states are the starting point for astatine–oxygen bonds: treating them with an oxygen-containing oxidizer leads to the formation of astatate ions, AtO−
3. Further oxidation, e.g., by hypochlorite or electrochemical oxidation, was originally thought to form unstable astatine(VII), either as perastatic acid H₅AtO₆ (analogous to periodic acid) or perastatate AtO−
4; however, this has never been confirmed. The intermediate states coprecipitate with several silver(I), thallium(I) or caesium oxygen-containing salts (oxysalts), such as thallium(I) dichromate or silver(I) iodates, to form cationic astatine. Astatine may also replace a hydrogen atom in benzene to form C₆H₅At, which may be oxidized to C₆H₅AtCl₂ by chlorine; by treating this compound in an alkaline solution of hypochlorite, C₆H₅AtO₂ may be produced. Astatine may form bonds to the other chalcogens, such as S₇At⁺ and At(CSN)−
2 with sulfur, a coordination selenocarbamide compound with selenium, and astatine–tellurium colloid with tellurium. Additionally, astatine is known to bind to nitrogen, lead, and boron under the proper conditions.

Isotopes

Main article: Isotopes of astatine

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

Alpha decay characteristics for sample astatine isotopes

Mass number	Mass excess	Mass excess daughter	Average energy of alpha decay	Half-life	Probability of alpha decay	Alpha half-life
207	−13.243 MeV	−19.116 MeV	5.873 MeV	1.80 h	8.6%	28.4 h
208	−12.491 MeV	−18.243 MeV	5.752 MeV	1.63 h	0.55%	17.1 d
209	−12.880 MeV	−18.638 MeV	5.758 MeV	5.41 h	4.1%	7.5 d
210	−11.972 MeV	−17.604 MeV	5.632 MeV	8.1 h	0.175%	267 d
211	−11.647 MeV	−17.630 MeV	5.983 MeV	7.21 h	41.8%	21.3 h
212	−8.621 MeV	−16.436 MeV	7.825 MeV	0.31 s	≈100%	0.31 s
213	−6.579 MeV	−15.834 MeV	9.255 MeV	125 ns	100%	125 ns
214	−3.380 MeV	−12.366 MeV	8.986 MeV	558 ns	100%	558 ns
219	10.397 MeV	4.073 MeV	6.324 MeV	56 s	97%	58 s
220	14.350 MeV	8.298 MeV	6.052 MeV	3.71 min	8%	1.05 h
221	16.810 MeV	11.244 MeV	5.566 MeV	2.3 min	experimentally alpha stable	∞

Alpha decay energy follows 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. However, ²²¹At has a significantly higher energy than the previous isotope; 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 time as the previous isotope (8.1 hours for ²¹⁰At and 7.2 hours for ²¹¹At), 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 ²¹³At releasing the highest amount of 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; this happens due to the increasing role of beta decay. This decay mode is especially important for astatine: as early as 1950, it was postulated that the element has no beta-stable isotopes (i.e., those that do not beta decay at all). The beta decay mode has been found for all astatine isotopes except for ²¹³At, ²¹⁴At, ²¹⁵At, ^216mAt, and ²¹⁷At.

The most stable of them is ²¹⁰At, which has a half-life of 8.1 hours. This isotope's primary decay mode is beta decay to a relatively long-lived (compared to astatine isotopes) alpha emitter, polonium-210. In total, only five isotopes have half-lives exceeding one hour, namely those between 207 and 211. The least stable ground state isotope is ²¹³At, with a half-life of 125 ns. It alpha decays to the extremely long-lived (in practice, stable) bismuth-209.

Astatine has 23 nuclear isomers (nuclei with one or more nucleons – protons or neutrons – in an excited state). A nuclear isomer may also be called a "meta-state"; this means 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 them is ^202mAt, which has a half-life of about 3 minutes; this is longer than those of all ground states except those of isotopes 203–211 and 220. The least stable one is ^214mAt; its half-life of 265 ns is shorter than those of all ground states except that of ²¹³At.

Natural occurrence

Astatine is the rarest naturally occurring element that is not a transuranic element, with the total amount in Earth's crust estimated to be less than 28 grams (0.99 oz) at any given time. Astatine present at the formation of the Earth has long since decayed; all natural astatine at present has formed through the decay of heavier elements. Previously thought to be the rarest element occurring on the Earth, astatine 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; these are ²¹⁴At to ²¹⁹At. Because of their short half-lives, they are found only in trace amounts. There is no data indicating astatine occurrence in stars.

Neptunium series, showing the decay products, including ²¹⁷At, formed from neptunium-237

Four out of these isotopes (²¹⁵At, ²¹⁷At, ²¹⁸At, and ²¹⁹At) are found there due to their production in major natural decay chains. The father isotope of ²¹⁹At, francium-223, alpha decays with a probability of only 0.006%, making this astatine isotope extremely rare even compared to other astatine isotopes, although its half-life is the longest of the natural astatine isotopes at 56 seconds. This rare isotope decays to polonium-215, which itself beta decays to ²¹⁵At with an even smaller probability of 0.00023%; for this reason, the Americas to a depth of 16 kilometers (10 mi) contain only a trillion ²¹⁵At atoms at any given time. ²¹⁸At 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. Therefore, most of Earth's astatine is ²¹⁷At, whose father (francium-221) decays exclusively to this nuclide; its fathers, grandfathers, so on, decay exclusively to one nuclide, to give only one possible way for the starting nuclide in the neptunium series, neptunium-237, to decay – via producing ²¹⁷At.

The other remaining isotopes (²¹⁴At and ²¹⁶At, as well as ²¹⁵At) are found as the result of triple alpha decay of the naturally present protactinium isotopes ²²⁶Pa, ²²⁷Pa, and ²²⁸Pa. However, these are extremely rare, so much so that they are often not even cited as natural astatine isotopes.

Synthesis

Formation

Possible reactions after bombarding bismuth-209 with alpha particles

Reaction	Energy of alpha particle
209 83Bi + 4 2He → 211 85At + 2 1 0	26 MeV
209 83Bi + 4 2He → 210 85At + 3 1 0	40 MeV
209 83Bi + 4 2He → 209 85At + 4 1 0	60 MeV

Astatine was first produced by bombarding bismuth-209 with energetic alpha particles; this is still the major route used to create the relatively long-lived isotopes ²⁰⁹At through ²¹¹At. Astatine is only produced in microscopic quantities, with modern techniques allowing production runs of 2 terabecquerel (about 25 μg).

The most important isotope is now ²¹¹At, that being the only one to find a commercial use. To produce the bismuth target, the metal is sputtered on a gold, copper, or aluminum surface, to form a 50–100 milligrams per centimeter squared bismuth layer (or, alternatively, bismuth oxide is forcibly fused with a copper plate). The target is kept under a chemically neutral N₂ atmosphere and is cooled with water to prevent premature astatine vaporization. In a particle accelerator such as a cyclotron, alpha particles are collided with bismuth. Even though there is only one bismuth isotope used, bismuth-209, the reaction may occur in three possible ways, producing ²⁰⁹At, ²¹⁰At, and ²¹¹At. In order to eliminate the undesired nuclides, the maximum energy of the particle accelerator is set to 28 MeV.^{[citation needed]}

Separation

Since the element is the main product of the synthesis, after its formation it must only be separated from the target and the traces of other radioisotopes. The target (with astatine in it) is heated to 270 °C (520 °F) to vaporize away the volatile traces of various radioisotopes, after which the temperature is raised to 800 °C (1450 °F). 80% of astatine may vaporize at this temperature, but bismuth begins to vaporize as well. Astatine's vaporization does not occur at an adequate rate under temperatures below 600 °C (1100 °F); at temperatures below 800 °C (1450 °F), astatine's volatility from a bismuth surface decreases significantly. The condensed vapor (distillate) is collected on a water-cooled surface, which is put in a U-like quartz vessel. The vessel is heated to 130 °C (270 °F) to remove the further traces of impurities (namely polonium) and then to 500 °C (930 °F) to remove astatine, which is collected on a cold finger. The element is then washed off the cold finger with a weak nitric acid solution. Using this technique, up to a 30% yield of astatine may result.

Uses and precautions

Several ²¹¹At-containing molecules and their uses

Agent	Applications
[211At]astatine-tellurium colloids	Compartmental tumors
6-[211At]astato-2-methyl-1,4-naphtaquinol diphosphate	Adenocarcinomas
211At-labeled methylene blue	Melanoma
Meta-[211At]astatobenzyl guanidine	Neuroendocrine tumors
5-[211At]astato-2'-deoxyuridine	Various
211At-labeled biotin conjugates	Various pretargeting
211At-labeled octeotide	Somatostatin receptor
211At-labeled mAbs and fragments	Various
211At-labeled biophosphonates	Bone metastases

The newly formed ²¹¹At is important in nuclear medicine. Once produced, astatine must be used quickly, as ²¹¹At decays with a half-life of 7.2 hours; however, this is long enough to permit multi-step labeling strategies. ²¹¹At can be used for targeted alpha particle radiotherapy, since it decays either via alpha decay to bismuth-207, or via electron capture to an extremely short-lived nuclide of polonium-211, which itself alpha decays.

Similarly to iodine, astatine is collected by the thyroid gland, although to a lesser extent; however, it concentrates in the liver if released to the body, in the form of a radiocolloid. The principal medicinal difference between ²¹¹At and iodine-131 (a radioactive iodine isotope, also used in medicine) is that astatine does not destroy the neighboring parathyroid gland, as it does not emit beta particles: an average alpha particle released by ²¹¹At travels about 70 µm, while a beta particle emitted by iodine-131 travels about 2 mm.

Because of its short half-life and particle run, astatine is considered preferable to iodine-131 in the diagnosis of diseases. However, it attacks the thyroid gland much more strongly, and in rats and monkeys a repetitive nuclide injection caused tissue destruction in the gland, followed by dysplasia; this should be true for all organisms with a thyroid. When lethal quantities are injected, morphological changes in other tissues are not found, with the possible exception of the breasts.

Bibliography

• 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. 
• Lavrukhina, A. K.; Pozdnyakov, A. A. (1966) (in Russian). Аналитическая химия технеция, прометия, астатина и франция [Analytical Chemistry of Technetium, Promethium, Astatine, and Francium]. Nauka.