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, 3, 5, 7
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 210At trace 8.1 h ε, β+ 3.981 210Po α 5.631 206Bi 211At syn 7.2 h
· r

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 decay of heavier elements, and decays away rapidly, so much less is known about this element than its upper neighbors in the periodic table. However, research has shown this element follows periodic trends, being the heaviest known halogen, with melting and boiling points being higher than those of lighter halogens.

Astatine was first produced by Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè in 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 (1 oz) at 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, but the most stable astatine-210 is not. The second longest-living astatine-211 is the only one to find a commercial use, being useful as an alpha emitter in medicine; however, only extremely small quantities are used, and in larger ones it is very hazardous, as it is intensely radioactive.

Until recently most of the chemical characteristics of astatine were inferred from comparison with other elements; however, important studies have already 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 is known to form ionic astatides with metals. Bonds to nonmetals result in positive oxidation states, with +1 best portrayed by monohalides and their derivatives, while the higher are characterized by bond to oxygen and carbon. Attempts to synthesize astatine fluoride have been met with failure.

Contents 1 General characteristics 2 Isotopes 3 History 4 Natural occurrence 5 Chemical reactivity and compounds 6 Production 6.1 Separation 6.1.1 Distillation 6.1.2 Chemical routes 7 Uses and precautions 8 See also 9 Notes 10 References 11 Bibliography

General characteristics

Astatine is a highly radioactive element: its isotopes have their half-lives under half a day, decaying into those of bismuth, polonium, radon, or 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.^[1]

Astatine is a halogen,^[2] though it is more metallic than iodine. Research into astatine is limited by the very small quantities available, which is a consequence of its extremely short half-life;^[3] however, it has been shown that astatine suits most periodic trends very well. Like other halogens, it is composed of diatomic At₂ molecules on standard conditions.^[4] Its melting and boiling point are estimated about 300 °C (575 °F) and 370 °C (700 °F), respectively. This allows astatine to fit well the trend in increase in melting and boiling points with the atomic number among halogens.^[3]

The element is often cited to have the electronegativity of 2.2 (Pauling scale), as this is stated in Pauling's work,^[5] lower than that of iodine (2.5 in the original work and 2.66 now^[6]) and same as hydrogen; the experiments have shown that the actual astatine electronegativity is slightly below that of hydrogen^[7] (see below). It sublimes more readily than iodine, with lower vapor pressure; it also dissolves in water.^[3]

Isotopes

There are 32 known isotopes of astatine, with atomic masses of of 191 and 193–223.^[1] No stable or at least long-lived astatine isotope is known, and no such an isotope is expected to exist.^[8]

Alpha decay energy follows the same trend as other heavy elements.^[8] Lighter astatine isotopes have quite high energies of alpha decay, which get lower as the nuclei get heavier. But astatine-211, the nucleus with 126 neutrons (126 is a magic number and corresponds to a filled neutron shell) has a significantly higher energy than the previous one. Despite having a similar half-life time with the previous isotope (8.1 hours for ²¹⁰At and 7.2 hours for ²¹¹At), the alpha decay probability is way higher for the latter: 41.81% against only 0.18%.^{[1][note 2]} The two following isotopes release even more energy, with astatine-213 releasing the highest amount of energy of all astatine isotopes. For this reason, it is the shortest-lived astatine isotope.^[8] Even though heavier astatine isotopes release less energy, no long-lived astatine isotope exists; this happens due to increasing role of beta decay.^[8] This decay mode is especially important for astatine: as early as 1950, it has been postulated that the element has no beta-stable isotopes (i.e. those that do not beta decay at all).^[9] The mode has already been found for all astatine isotopes, except for ²¹³At, ²¹⁴At, ²¹⁵At, ^216mAt, and ²¹⁷At.^[1]

The most stable of them is astatine-210, which (as it has been mentioned) 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 with mass numbers between 207 and 211. The least stable ground state isotope is astatine-213, with a half-life of 125 ns. It alpha decays to the extremely long-lived (in practice, stable) bismuth-209.^[1]

In nature, six astatine isotopes are present: astatine-214 to -219; they decay rapidly by primarily alpha decay, but are rapidly recreated by the decay of heavier elements as well. Astatine-215, -217, -218, and -219 are present in the major decay chains,^[10] while astatine-214 and -216 (as well as astatine-215) occur as result of decay of natural protactinium.^[8]

Astatine has 23 nuclear isomers (nuclei with of one or more nucleons (protons or neutrons) excited. A nuclear isomer may also be called a "meta state," which 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 it, not always one isomer for one isotope. The most stable of them is astatine-202m1, which has a half-life of about 3 minutes; this is longer than those of all ground states, except for isotopes with mass numbers 203–211 and 220. The least stable one is astatine-214m1; its half-life of 265 ns is shorter than those of all ground states, except for astatine-213.^[1]

History

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 there could be the fifth halogen. Before 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).^[11] Scientists tried to find it in nature, which led to erroneous discoveries.^[12]

The first claimed discovery of eka-iodine was made by Fred Allison and associates at the Alabama Polytechnic Institute (now Auburn University) in 1931; the discoverers named element 85 "alabamine" and assigned it the symbol Ab, which were used for a few years.^[13][14][15] In 1934, however, H. G. MacPherson of University of California, Berkeley disproved the effectiveness of Allison's device and the validity of this discovery.^[16] 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.^[17]

In 1940, the Swiss chemist Walter Minder announced the discovery of element 85 as the product of the beta decay of Radium A (polonium-218), choosing the name "helvetium" (from Helvetia, "Switzerland"). However, Berta Karlik and Traude Bernert were unsuccessful in reproducing his experiments, attributing the results to contamination of his radon stream.^[18] (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,"^[19] but Karlik and Bernert were again unable to reproduce these results.^[20]

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 actually created it by bombarding bismuth with alpha particles.^[21] 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),^[21] and the ending "-ine," found in the names of the four previously discovered halogens. Three years later, astatine was found as a product of natural decay chains by Karlik and Bernert.^[22][23] Since then, astatine has been found in three out of the four natural decay chains.^[24]

Natural occurrence

Astatine is the rarest naturally occurring element among those that do not belong to the transuranium elements series, with the total amount in Earth's crust estimated to be less than 28 grams (1 oz) at given time.^[25] Astatine present at the formation of the Earth has long since decayed; 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.^[26]

Six astatine isotopes occur naturally: these are astatine-214 to -219.^[27] Because of their short half-lives, they are found only in trace amounts.^[10] Although astatine is found on the Earth, there is no data of astatine occurrence in stars.^[28]

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 astatine-219, 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.^[1] This rare isotope decays to polonium-215, which itself beta decays to astatine-215 with an even smaller probability of 0.00023%; for this reason, the Americas to a depth of sixteen kilometers (10 mi) contains only a trillion astatine-215 atoms at given time.^[29] Astatine-218 is found in nature as result of polonium-218 beta decay; like francium-223 and polonium-215, decay to an astatine isotope is not the primary decay mode.^[10] Therefore, most Earth's astatine is astatine-217, whose father (francium-221) decays exclusively to this nuclide; its fathers, grandfathers, so on, decay to exclusively to one exact nuclide, to make only one possible way for the starting nuclide in the neptunium series, neptunium-237, to decay to astatine-217.^[10]

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.^[27] These, however, are extremely rare, so they are often even not cited as natural astatine isotopes.^[3][30]

Chemical reactivity and compounds

Astatine is the least reactive of the halogens, being less reactive than iodine;^[31] 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.^[20][32]

The most common compound of the element is hydrogen astatide.^[3] 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 when compared to hydrogen (unlike the other halogens), the molecule should more properly be called astatine monohydride;^[7] this reversal of polarity partially explains its lower stability compared to the other hydrogen halides. As it is easily oxidized, it is precipitated in aqueous nitric acid/silver(I) solution, forming silver(I) astatide, AgAt.^[3]

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.^[2] 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, a iodine/iodine monobromide/bromide solution. The excess of iodides or bromides may lead to AtBr−
2 and AtI−
2 ions;^[2] in a chloride solution, they may turn to species like AtCl−
2 or AtBrCl⁻ via equilibrium.^[33] 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.^[2] 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 camera where astatine is situated, supporting the existence of stable neutral molecules in the plasma ion state.^[2] No astatine fluoride has not 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 characterization of radon fluorides.^[2]

The lower oxidation states are the starting point for astatine–oxygen bonds:^{[note 3]} treating them with an oxygen-containing oxidizer leads to the formation of astatate ions, AtO−
3.^[35] 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;^[3] however, this has never been confirmed.^[36] The intermediate states coprecipitate with several silver(I), thallium(I) or caesium oxysalts to form cationic astatine.^[35] 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.^[35] 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.^[37] Additionally, astatine is known to bind to nitrogen,^[38] lead,^[39] and boron.^[40]

Production

Astatine was first produced by bombarding bismuth with energetic alpha particles; this is still the major route used to create the relatively long-lived isotopes astatine-209 through -211. Astatine is only produced in microscopic quantities, with modern techniques allowing production runs of 2 terabecquerel (about 25 μg).^[42]

The most important isotope is now astatine-211, being the only one to find a commercial use.^[43] 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 pressed into a copper plate).^[44] The target is kept under a chemically neutral nitrogen atmosphere^[45] and is cooled with water to prevent the premature astatine vaporization.^[44] In a particle accelerator,^[46] such as a cyclotron, alpha particles are collided with bismuth. Even though bismuth is composed of only one isotope, bismuth-209, the reaction may occur in three possible ways, producing astatine-209, -210, and -211. In order to eliminate the undesired nuclides, the maximum energy of the reaction is set to 28 MeV. The produced astatine is then easily separated, as astatine sets itself free and a stream of the element is collected as a deposit on a glass cold finger.^[45]

Separation

The freshly formed astatine is separated either chemically or via distillation.^[47]

Distillation

Astatine is very volatile, even though less than iodine (in acidic solutions).^{[note 4]} Out of such solutions, the iron(II) sulfate/sulfuric acid solution seems the most promising: it may provide the distillation of 90% of the element (85% of the solution distilled).^[49]

If dry distillation is used, the element (after its formation) must be separated from the target and the traces of other radioisotopes. The target is heated to 300–600 °C (575–1100 °F). The freshly vaporized astatine (in form of a stream) is collected as a deposit on a glass cold finger.^[49] To eliminate the polonium content in astatine, it is re-distilled.

However, astatine is thought to react with bismuth to form non-volatile compounds. This is likely to cause the low astatine outcome (up to 15%).^[49]. If the temperature is raised to 700–800 °C (1275–1450 °F), 80% of astatine may be distilled, which may be explained by suggesting that the non-volatile astatine compounds decompose; however, bismuth begins to vaporize as well. Mixing the methods, up to 30% of astatine may result.^[49]

Chemical routes

Astatine may be extracted from acidic aqueous solutions using organic solvents. The distribution factor varies on the solvent: the highest one is 200, found in 0.01 M nitric acid/benzene system (the second highest is 91, in 0.01 M nitric acid/tetrachloromethane system).^[50] In general, it is similar to iodine in that it dissolves in benzene, carbon disulfide, and tetrachloromethane (therefore, they have high distribution factors in systems with water), but, unlike iodine, it cannot be extracted from alkaline solutions of organic solvents, as it reacts with bases and disproportionates.^[3] Diatomic interhalogen molecules (with iodine and bromine) have smaller distribution factors than dihalogens (where both atoms are the same), as the former are dipoles.^[50] Separation from other elements is done via extraction from hydrochloric acid/isopropyl ether solutions. Iron(III) hydroxide may be used to remove the further traces. Using 8 M HCl solutions, outcome may be as high as 90%.^[50]

When astatine is introduced into negatively charged chloro complexes and dissolved in chloride or hydrogen chloride (at best at 5–8 M, because the complex decomposes on a lower chloride concentration), such a complex can be sorbed on a cation resin; the same is true for the astatine cation.^[51] Such astatine is originally inserted into tellurium, which is (together with polonium impurity) later washer away with hydrochloric acid/chlorine solution. Astatine is later desorbed via chloric water. The cation resin is processed with nitric acid solution (with a small dichromate content), and the solution is introduced to it, and later to nitric acid solution (to remove the chloride ions). Thereafter, astatine is desorbed with nitric acid/dichromate solution.^[52]

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 adsorbation on the surface of the precipitate.^[53] The adsorbation is suppressed by increasing the acidity of the solution, washing off the precipitate with acetone, or adding iodine; the latter proves the adsorbation character of the astatine coprecipitation.^[53] The most important such a reaction is the one with tellurium, which is catalyzed by reducers like tin dichloride; however, the reaction does not occur in alkaline solutions. The amount of astatine precipitated does not rely on tellurium amount, reaching 90% in concentrated HCl (this reaction also eliminates different impurities).^[53] Astatide and astatate have been shown to coprecipitate with iodide and iodate; they, however, cannot be washed off with acetone.^[54] These methods are used most typically only when astatine is the result of a different, way rarer reaction of bismuth, lead, or thorium with high-energy protons.^[54] Since such reactions produces hundreds less times astatine than the one involving bismuth and alpha particles. Therefore, the method used of separation should be effective; this one has proved to be so.^[54]

Uses and precautions

The newly formed astatine-211 is important in prostate cancer treatment. Once produced, astatine should be used quickly, as astatine-211 decays with a half-life of 7.2 hours; this, however, is long enough to permit multi-step labeling strategies. Astatine-211 can be used for targeted alpha particle radiotherapy, since it decays either via alpha decay to bismuth-207, or via electronic capture to an extremely short-lived nuclide of polonium-211, which itself alpha decays.^[55] A selection of astatine-containing organic compounds and their medical applications can be seen in the table below:

Similarly to iodine, astatine is collected by the thyroid gland, even though to a lesser extent; however, it concentrates in the liver if released to the body if the form of a radiocolloid.^[44] The principled behavior difference between astatine-211 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 astatine-211 runs about 70 µm, while a beta particle emitted by iodine-131 runs about 2 mm.^[44] Thanks to its short half-life and small particle run, it is preferable to be used in diagnosis of diseases (compared to iodine-131).^[44] However, it attacks the thyroid gland much stronger, and the repetative injection of the nuclide caused tissue destruction in the gland, followed by dysplasia in rats and monkeys. When lethal quantities are added, morphological changes in other tissues are not found, with the possible exception of the breasts.^[56]

See also

Books View or order collections of articles	Astatine Period 6 elements Halogens Chemical elements (sorted alphabetically) Chemical elements (sorted by number)
Portals Access related topics	Chemistry portal
Find out more on Wikipedia's Sister projects	[//commons.wikimedia.org/wiki/Special:Search/Astatine Images and media] from Commons [//en.wiktionary.org/wiki/Special:Search/astatine Definitions] from Wiktionary [//en.wikiversity.org/wiki/Special:Search/Astatine_atom Learning resources] from Wikiversity

Notes

1. ^ In the table, under words "mass excess," the energy equivalents are given rather than the real mass excesses; "mass excess daughter" stands for energy equivalent of sum of mass excesses of the daughter of the isotope and the alpha particle; "alpha half-life" refers to the half-life where decay modes other than alpha are omitted.
2. ^ This means that if decay modes other than alpha are omitted, then astatine-210 has the alpha half-life of 3116.4 hours (129.8 days), and astatine-211 has 21.3 hours (0.9 days). Therefore, astatine-211 is way less stable toward alpha decay than the previous isotope.
3. ^ This, however, does not mean that there are no halogen-based astatine complexes in an oxidation state above +1, as well as that there is no oxygen-based astatine species in the oxidation state of +1. Moreover, it is sure that the hypoastatite ion, AtO⁻, exists;^[3] it is likely that the tetrachloroastatate ion, AtCl−
   4, exists as well.^[34]
4. ^ For example, at the room temperature, half of astatine vaporizes in an hour if put on clean glass. However, if put on gold or platinum, the half-vaporization period grows to 16 hours; this may be caused by interaction between astatine and the metal.^[48]

References

Bibliography

• Zuckerman, J. J.; Hagen, A. P. (1989). Inorganic Reactions and Methods, the Formation of Bonds to Halogens. John Wiley and Sons. ISBN 0471186562. 
• (Russian) Lavrukhina, A. K.; Pozdnyakov, A. A. (1966). Аналитическая химия технеция, прометия, астатина и франция (Analytical Chemistry of Technetium, Promethium, Astatine, and Francium). Nauka. 

Diatomic chemical elements Hydrogen H2 Nitrogen N2 Oxygen O2 Fluorine F2 Chlorine Cl2 Bromine Br2 Iodine I2 Astatine At2

H																															He
Li	Be																									B	C	N	O	F	Ne
Na	Mg																									Al	Si	P	S	Cl	Ar
K	Ca															Sc	Ti	V	Cr	Mn	Fe	Co	Ni	Cu	Zn	Ga	Ge	As	Se	Br	Kr
Rb	Sr															Y	Zr	Nb	Mo	Tc	Ru	Rh	Pd	Ag	Cd	In	Sn	Sb	Te	I	Xe
Cs	Ba	La	Ce	Pr	Nd	Pm	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Hf	Ta	W	Re	Os	Ir	Pt	Au	Hg	Tl	Pb	Bi	Po	At	Rn
Fr	Ra	Ac	Th	Pa	U	Np	Pu	Am	Cm	Bk	Cf	Es	Fm	Md	No	Lr	Rf	Db	Sg	Bh	Hs	Mt	Ds	Rg	Cn	Uut	Uuq	Uup	Uuh	Uus	Uuo
Alkali metals Alkaline earth metals Lanthanides Actinides Transition metals Post-transition metals Metalloids Other nonmetals Halogens Noble gases Unknown chem. properties
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