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Astatine Information

Astatine (/ ˈ æ s t ə t iː n / as-tə-teen or / ˈ æ s t ə t ɪ n / 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. Earlier studies have shown this element follows periodic trends, 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 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. 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.

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 as the result of decay of heavier elements; however, the most stable astatine-210 and the industrially used astatine-211 are not.

Contents

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] Its extremely short half-life allows only production in microscopic quantities, which limits research into astatine.[2]

Astatine is often cited as either a nonmetal[3] or a metalloid.[4] Its most common properties are normal for a heavier (more metallic than iodine) halogen:[5] Like other halogens, it is composed of diatomic At2 molecules on standard conditions.[6] Its melting and boiling point suit the trend in halogen series, increasing with the increase in atomic number, measured to be 302 °C (576 °F) and 337 °C (639 °F), respectively.[7] Like iodine, it is also very volatile: at the room temperature, half of astatine vaporizes in an hour if put on clean glass.[note 1] Most chemical properties, such as formation of the astatide anion, are in line with other halogen as well.[2] However, it has a few metallic properties: it is allocated on a cathode, forms a cation in strong acidic solutions, and so on.[9]

The element is often cited to have the electronegativity of 2.2 (Pauling scale), as this is stated in Pauling's work,[10] lower than that of iodine (2.5 in the original work and 2.66 now[11]) and same as hydrogen; however, experiments have shown that the actual astatine electronegativity is slightly below that of hydrogen[12] (see below). It sublimes more readily than iodine, with lower vapor pressure; it also dissolves in water.[2]

Isotopes

Main article: Isotopes of astatine

There are 32 known isotopes of astatine, with atomic masses of 191 and 193–223.[1] No stable or at least long-lived astatine isotope is known, and no such isotope is expected to exist.[13]

Alpha decay characteristics for several astatine isotopes[note 2]
Mass number Mass excess [1] Mass excess daughter[1] Average energy of alpha decay Half-life[1] Probability of alpha decay[1] 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[note 3] 16.810 MeV 11.244 MeV 5.566 MeV 2.3 min experimentally alpha stable ∞

Alpha decay energy follows the same trend as other heavy elements.[13] Lighter astatine isotopes have quite high energies of alpha decay, which get lower as the nuclei get heavier. However, 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 much higher for the latter: 41.81% against only 0.18%.[1][note 4] 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.[13] Even though heavier astatine isotopes release less energy, no long-lived astatine isotope exists; this happens due to increasing role of beta decay.[13] 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).[14] 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]

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. There may be more than 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

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 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).[15] Scientists tried to find it in nature, which led to erroneous discoveries.[16]

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.[17][18][19] In 1934, however, H. G. MacPherson of University of California, Berkeley disproved the effectiveness of Allison's device and the validity of this discovery.[20] 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.[21]

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 (radon-222 is the parent isotope of polonium-218).[22] 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,"[23] but Karlik and Bernert were again unable to reproduce these results.[24]

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 actually created it by bombarding bismuth with alpha particles.[25] 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),[25] 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.[26][27] Since then, astatine has been found in three out of the four natural decay chains.[28]

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.[29] 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.[30]

Six astatine isotopes occur naturally: these are astatine-214 to -219.[31] Because of their short half-lives, they are found only in trace amounts.[32] Although astatine is found on the Earth, there is no data of astatine occurrence in stars.[33]

Neptunium series, showing the decay products, including astatine-217, 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 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.[34] 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.[32] 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.[32]

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.[31] These, however, are extremely rare, so they are often even not cited as natural astatine isotopes.[2][35]

Chemical reactivity and compounds

Astatine is the least reactive of the halogens, being less reactive than iodine;[36] 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.[24][37]

The most common compound of the element is hydrogen astatide.[2] 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;[12] 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.[2]

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.[5] 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;[5] in a chloride solution, they may turn to species like AtCl− 2 or AtBrCl− via equilibrium.[38] 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.[5] 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.[5] 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 characterization of radon fluorides.[5]

The lower oxidation states are the starting point for astatine–oxygen bonds:[note 5] treating them with an oxygen-containing oxidizer leads to the formation of astatate ions, AtO− 3.[40] 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;[2] however, this has never been confirmed.[41] The intermediate states coprecipitate with several silver(I), thallium(I) or caesium oxysalts to form cationic astatine.[40] 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.[40] 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.[42] Additionally, astatine is known to bind to nitrogen,[43] lead,[44] and boron.[45]

Production

Formation

Possible reactions after bombarding bismuth with alpha particles
Reaction Energy of alpha particle
209 83Bi + 4 2He → 211 85At + 2 1 0n 26 MeV[24]
209 83Bi + 4 2He → 210 85At + 3 1 0n 40 MeV[24]
209 83Bi + 4 2He → 209 85At + 4 1 0n 60 MeV[46]

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).[47]

The most important isotope is now astatine-211, being the only one to find a commercial use.[48] 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).[49] The target is kept under a chemically neutral nitrogen atmosphere[50] and is cooled with water to prevent the premature astatine vaporization.[49] In a particle accelerator,[51] 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.

Separation

Since the element is the main product of the synthesis, after its formation, it only must be separated from the target and the traces of other radioisotopes.[52] 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 be distilled, but bismuth begins to vaporize as well.[52] Its vaporization does not occur on temperatures below 600 °C (1100 °F), but on these temperatures, astatine volatility from bismuth surface decreases significantly.[note 6] The 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.[52] The element is then washed off it with a weak nitric acid solution. Using this technique, up to 30% of astatine may result.[52]

Uses and precautions

Several astatine-211-containing molecules and their uses[53]
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 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.[53]

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.[49] 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.[49]

Because of its short half-life and small particle run, astatine is considered preferable to iodine-131 in the diagnosis of diseases.[49] However, it attacks the thyroid gland much more strongly, and the repetitive injection of the nuclide caused tissue destruction in the gland, followed by dysplasia in rats and monkeys; this should be true for all living organisms.[54] When lethal quantities are added, morphological changes in other tissues are not found, with the possible exception of the breasts.[9]

See also

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Notes

  1. ^ 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.[8]
  2. ^ 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.
  3. ^ Since astatine-221 has not been shown to alpha decay, the energy of alpha decay is theoretical. Also note that mass excess for the nuclide was not measured, but theoretically calculated instead.
  4. ^ This means that if decay modes other than alpha are omitted, then astatine-210 has the alpha half-life of 6,413.8 hours (267.2 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.
  5. ^ 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;[2] it is likely that the tetrachloroastatate ion, AtCl− 4, exists as well.[39]
  6. ^ Astatine is thought to react with bismuth at lower temperatures to form non-volatile compounds, which prevents the former from vaporization; these compounds are believed to decompose at 700–800 °C (1275–1450 °F).[52]

References

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  51. ^ Stigbrand, Torgny; Carlsson, Jorgen; Adams, Gregory P. (2008). Targeted Radionuclide Tumor Therapy: Biological Aspects. Springer. p. 150. ISBN 9781402086953.
  52. ^ a b c d e Lavrukhina & Pozdnyakov 1966, p. 254.
  53. ^ a b Vértes, Attila; Nagy, Sándor; Klencsár, Zoltán (2003). Handbook of Nuclear Chemistry. 4. Springer. p. 337. ISBN 1402013167.
  54. ^ Cobb, L.M.; Harrison, A.; Butler, S.A. (1988). "Toxicity of astatine-211 in the mouse". Human & Experimental Toxicology 7 (6): 529. doi:10.1177/096032718800700602.

Bibliography

Diatomic chemical elements
Periodic table
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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