Astatium (Astatium), At (From the Greek αστατος - unstable) - a radioactive chemical element of group VII periodic system elements, atomic number 85, mass number of the longest-lived isotope 210. Astatine is the heaviest element of the halogen group.
Astatine under the name ekaioda was predicted by D. I. Mendeleev. First received by D.Corson, K. McKenzie and E. Segre in 1940. In nature, astatine was first discovered in 1943 by the Austrian scientists Karlik and Bernert. It is part of the natural radioactive series (the most stable of them 219 At).
Isotopes of astatine
The longest-lived isotopes 210 At (T=8.1 h, decays by K-capture (99%) and emits α-particles) and 211 At (T=7.21 h, decays by K-capture (59.1%) and emits α-particles). Note that 211 At has the ability known in radiochemistry as "branched decay". The essence of the phenomenon is that some of the atoms of this isotope undergo one type of decay, while others - another, and alpha particles are released as the final result of these decays.
There are 24 known isotopes of astatine with mass numbers from 196 to 219. The most important of them are: 209 At (T = 5.5 h), 210 At (T = 8.1 h) and 211 At (T = 7.2 h). All of these isotopes decay by electron capture and alpha decay and are the longest-lived isotopes of this element. They are obtained by irradiating bismuth with alpha particles according to the reaction equation 209 Bi (α, xn)At, as well as by irradiating thorium and uranium with high-energy protons. Metals or oxides of these elements pressed into copper substrates are used as the target material. The shortest-lived isotope of astatine is 214 At (2*10 -6 s). The mass activity of 211 At is 7.4⋅10 13 Bq/mg.
At are formed in extremely small quantities during the radioactive decay of uranium and thorium in natural conditions(0.02%). The surface layer of the earth's crust 1.6 km thick contains 70 mg of astatine. The constant presence of astatine in nature is due to the fact that its short-lived radionuclides (215 At, 218 At and 219 At) are part of the radioactive series 235 U and 238 U. The rate of their formation is constant and equal to the rate of their radioactive decay, therefore, in earth's crust contains constant numbers of these atoms. The total content of astatine in a layer of the earth's crust 1.6 km thick is estimated at 69 mg.
Physical and Chemical properties
Astatine has not been isolated in weight quantities; Experiments with microquantities of this element showed that astatine exhibits, on the one hand, the properties of a non-metal and is similar to iodine, on the other hand, the properties of a metal and is similar to polonium and bismuth (most likely astatine is still a metal). In chemical compounds, astatine can exhibit oxidation states -1, +1, +3, +5 and +7. The most stable of them is -1.
Astatine (At)
Atomic number 85
Appearance - black and blue radioactive crystals
atomic mass ( molar mass) 209.9871 amu (g/mol)
Melting point 575 K
Boiling point 610 K
Specific heat capacity of astatine at a temperature of 298 K Ср=139.55 J/(kg-K).
Astatine has neither isotopic carriers nor a sufficiently satisfactory specific carrier. Being the heaviest halogen, it must have the properties of the latter. However, the electropositive properties of astatine are more pronounced than those of iodine. The situation is complicated by the fact that the chemistry of trace amounts of iodine is very different from the chemistry of its macroquantities.
Like iodine, astatine sublimates (sublimes) when room temperature, soluble in organic solvents, concentrated in thyroid gland. How the pure metal astatine behaves is amazing: it sublimes in molecular form from aqueous solutions. None of the known elements has this ability. Astatine is easily extracted by organic solvent liquids and is easily extracted by them. In terms of volatility, it is slightly inferior to iodine, but can also be easily distilled off.
Gaseous astatine is well adsorbed on metals (Ag, Au, Pt). Desorption of astatine occurs when metals are heated to 500°C in air or in vacuum. Thanks to this, it is possible to isolate astatine (up to 85%) from the products of bismuth irradiation by vacuum distillation with absorption of astatine by silver or platinum. At (0) is sorbed on glass from dilute nitric acid solutions. The chemical properties of astatine are very interesting and peculiar; it is close to both iodine and polonium, i.e., it exhibits the properties of both a non-metal (halogen) and a metal. This combination of properties is due to the position of astatine in the periodic system: it is the heaviest (and therefore the most “metallic”) element of the halogen group. Like the halogens, astatine gives the insoluble salt AgAt; like iodine, it is oxidized to the pentapal state (salt AgAtO3 is similar to AgJO3). However, like typical metals, Astatine is precipitated by hydrogen sulfide even from a strong acid solutions, is displaced by zinc from sulfuric acid solutions, and is deposited on the cathode during electrolysis.
Obtaining and determining astatine
Astatine is obtained by irradiating metallic bismuth or thorium with high-energy α-particles, followed by separation of astatine by co-precipitation, extraction, chromatography or distillation.
In accordance with the methods of obtaining astatine, it has to be separated from large quantities irradiated bismuth, uranium or thorium, as well as fission and deep fission products. A bismuth target irradiated with α-particles contains practically no radioactive impurities of other elements. Therefore, the main task of separating astatine is reduced to the removal of macroquantities of bismuth from a molten target by distillation. In this case, astatine is either adsorbed from the gas phase on platinum or silver, or condenses on glass or frozen solutions, or is absorbed by sulfite or alkali solutions. Other methods for separating astatine from a bismuth target are based on the extraction or co-precipitation of astatine after the target has been dissolved.
The main method for separating astatine from irradiated uranium and thorium is gas thermal chromatography. In this case, astatine evaporates from the target during the combustion of metals in oxygen and is adsorbed from the gas flow on silver, gold, or platinum. Another method for separating astatine from thorium and uranium targets is its sorption on metallic tellurium from hydrochloric acid solutions in the presence of reducing agents, followed by desorption with a weakly alkaline solution. The advantage of the first method is its rapidity (the extraction time is only 10 min). At 310°, more than 85% of astatine is concentrated on silver. Chemical separation of astatine can be carried out by dissolving a bismuth target in acid, followed by precipitation of bismuth in the form of phosphate, which does not capture astatine. Of interest is also the extraction of elemental astatine with diisopropyl ether from a hydrochloric acid solution.
Description of the presentation on individual slides:
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"Rare chemical elements and their application" "Astat" Prepared by Julia Borzenkova Pupil 11B MBOU class Secondary School No. 5 in Novocherkassk
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Introduction Astatine - element main subgroup seventh group, sixth period of the periodic system chemical elements D. I. Mendeleev, with atomic number 85. Denoted by the symbol At (lat. Astatium). Radioactive. Most heavy element from known halogens. The simple substance astatine normal conditions- unstable crystals black-blue. The astatine molecule appears to be diatomic (formula At2). Astatine - poisonous substance. Inhaling it in very small amounts can cause severe irritation and inflammation. respiratory tract, and a large concentration leads to severe poisoning.
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Physical Properties Astatine is a beautiful blue-black solid appearance similar to iodine. It is characterized by a combination of properties of non-metals (halogens) and metals (polonium, lead and others). Like iodine, astatine dissolves well in organic solvents and is easily extracted by them. In terms of volatility, it is slightly inferior to iodine, but it can also easily sublimate. Melting point 302 °C, boiling point (sublimation) 337 °C.
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Chemical properties Astatine is characterized by low vapor pressure, slightly soluble in water, and better soluble in organic solvents. Astatine in aqueous solution is reduced by sulfur dioxide SO2; like metals, it precipitates even from strongly acidic solutions with hydrogen sulfide (H2S). It is displaced from sulfuric acid solutions by zinc (metal properties). Like all halogens, astatine forms an insoluble salt AgAt (silver astatide). It is able to oxidize to the At(V) state, like iodine (for example, AgAtO3 salt is identical in properties to AgIO3). Astatine reacts with bromine and iodine, and interhalogen compounds are formed - astatine iodide AtI and astatine bromide AtBr: Both of these compounds dissolve in carbon tetrachloride CCl4.
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Chemical properties Astatine dissolves in dilute hydrochloric and nitric acids. With metals, astatine forms compounds in which it exhibits an oxidation state of −1, like all other halogens (NaAt is sodium astatide). Like other halogens, astatine can replace hydrogen in a methane molecule to produce tetraastatmethane CAt4. In this case, first astatmethane CH3At, then diastatmethane CH2At2 and astatine form CHAt3 are formed. AT positive degrees oxidation of astatine forms an oxygen-containing form, which is conventionally designated as Atτ + (astatine-tau-plus).
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History Predicted (as "eka-iodine") by D. I. Mendeleev. In 1931, F. Allison and coworkers (Alabama Polytechnic Institute) reported the discovery of this element in nature and proposed the name alabamine (Ab) for it, but this result was not confirmed. Astatine was first obtained artificially in 1940 by D. Corson, C. R. Mackenzie, and E. Segre (University of California at Berkeley). To synthesize the 211At isotope, they irradiated bismuth with alpha particles. In 1943-1946, astatine isotopes were discovered in the composition of natural radioactive series. In Russian terminology, the element was originally called "astatine". The names "Helvetin" were also proposed (in honor of Helvetia - ancient name Switzerland) and "leptin" (from the Greek "weak, shaky"). The name comes from Greek word"astatos", which literally means "unstable". And the element fully corresponds to the name given to it: its life is short, the half-life is only 8.1 hours.
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Astatine in nature Astatine is the most rare element among all found in nature. The surface layer of the earth's crust 1.6 km thick contains only 70 mg of astatine. The constant presence of astatine in nature is due to the fact that its short-lived radionuclides (215At, 218At and 219At) are part of the 235U and 238U radioactive series. The rate of their formation is constant and equal to the rate of their radioactive decay, therefore, the earth's crust contains a relatively constant equilibrium amount of astatine isotopes.
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Isotopes As of 2003, 33 isotopes of astatine are known, as well as 23 metastable excited states of astatine nuclei. All of them are radioactive. The most stable of them (from 207At to 211At) have a half-life more than an hour(the most stable is 210At, T1/2=8.1 hours); however, three natural isotopes have a half-life of less than a minute. Basically, astatine isotopes are obtained by irradiating metallic bismuth or thorium with high-energy α-particles, followed by separation of astatine by co-precipitation, extraction, chromatography or distillation. Melting point 302 °C, boiling point (sublimation) 337 °C.
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Astatine isotopes Mass number Mass of the isotope relative to 16O Half-life Form and energy of radiation, MeV 202 - 43 s CD; α, 6.50 203 - 102 with CD; α, 6.35 203 420 s CD; α, 6.10 204 - 1500 s K-z 205 - 1500 s KDz; α, 5.90 206 - 0.108 days KDz 207 - 6480 s K-z (90%); α (10%), 5.75 208 - 0.262 with short-circuit 208 6120 with short-circuit (>99%), α (0.5%), 5.65 209 - 0.229 with short-circuit (95%),α (5%), 5.65; γ 210 - 0.345 days K-z (> 99%), α (0.17%), 5.519 (32%); 5.437 (31%); 5.355 (37%); γ, 0.25; 1.15; 1.40 211 05317 0.3 days K-z (59 1%); α (40.9%); 5.862 γ, 0.671 212 05675 0.25 s α 213 05929 - α, 9.2 214 06299 ~2*10-6 s α, 8.78 215 05562 10-4 s α, 8.00 216 06967 3*10- 4 with α, 7.79 217 07225 0.018 with α, 7.02 218 07638 1.5D2.0 with α (99%), 6.63; β (0.1%) 219 - 5.4 with α (97%), 6.27; β (3%)
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Application The first attempts to apply astatine in practice were made as early as 1940, immediately after obtaining this element. Employee group University of California found that astatine, like iodine, is selectively concentrated in the thyroid gland. Experiments have shown that the use of 211At for the treatment of thyroid diseases is more beneficial than radioactive 131I. Thyroid
Astatine, the fifth halogen, is the least common element on our planet, except, of course, for the transuranium elements. An approximate calculation shows that the entire earth's crust contains only about 30 g of astatine, and this estimate is the most optimistic. Item #85 stable isotopes no, and the longest-lived radioactive isotope has a half-life of 8.3 hours, i.e. not even half of the astatine received in the morning remains by the evening.
Thus, the name astatine - and in Greek αστατος means "unstable" - successfully reflects the nature of this element. What then can astatine be interesting for and is it worth studying it? It is worth it, because astatine (just like promethium, technetium and francium) was created by man in the full sense of the word, and the study of this element gives a lot of instructive information - primarily for understanding the patterns in changing the properties of the elements of the periodic system. Showing up in some cases metallic properties, and in others - non-metallic, astatine is one of the most peculiar elements.
Until 1962 in Russian chemical literature this element was called astatine, and now the name “astatine” has been assigned to it, and this, apparently, is correct: neither in Greek nor in Latin name this element (in Latin astatium) does not have the suffix "in".
The search for ecaiod
D. I., Mendeleev called the last halogen not only ecaiodine, but also halogen X. He wrote in 1898: KX, KXO 3, etc., what hydrogen bond it will be a gaseous, very fragile acid, that the atomic one will be ... about 215.
In 1920 German chemist E. Wagner again drew attention to the still hypothetical fifth member of the halogen group, arguing that this element must be radioactive.
Then began an intensive search for element No. 85 in natural objects.
In assumptions about the properties of the 85th element, chemists proceeded from its location in the periodic system and from data on the properties of the neighbors of this element according to the periodic table. Considering the properties of other members of the halogen group, it is easy to notice the following pattern: fluorine and chlorine are gases, bromine is already a liquid, and iodine is solid, which exhibits, although to a small extent, the properties of metals. Ecaiodine is the heaviest halogen. Obviously, it should be even more metal-like than iodine, and, having many of the properties of halogens, one way or another similar to its neighbor on the left - polonium ... Together with other halogens, ecaiodus, apparently, should be in the water of the seas, oceans , boreholes. They tried to look for it, like iodine, in seaweed, brines, etc. The English chemist I. Friend tried to find the current astatine and francium in waters of the Dead seas, in which, as was known, both halogens and alkali metals more than enough. To extract ecaiodine from a solution of chlorides, silver chloride was precipitated; Friend believed that the sediment would also carry away traces of the 85th element. However, neither X-ray spectral analysis nor mass spectrometry gave a positive result.
In 1932 the chemists Polytechnic Institute State of Alabama (USA), headed by F. Allison, reported that they had isolated a product from monazite sand, which contained about 0.000002 g of one of the compounds of element No. 85. In honor of their state, they named it "alabamium" and even described its combination with hydrogen and oxygen-containing acids. The name alabamium for element 85 appeared in chemistry textbooks and reference books until 1947.
However, soon after this message, several scientists had doubts about the reliability of Allison's discovery. Properties of alabamium diverged sharply from predictions periodic law. In addition, by this time it became clear that all elements heavier than bismuth do not have stable isotopes. Assuming the stability of element No. 85, science would be faced with an inexplicable anomaly. Well, if element No. 85 is not stable, then it can be found on Earth only in two cases: if it has an isotope with a half-life greater than the age of the Earth, or if its isotopes are formed during the decay of long-lived radioactive elements.
The suggestion that element #85 could be a radioactive decay product of other elements has become Starting point for another large group researchers involved in the search for ecaiod. The first in this group should be called the famous German radiochemist Otto Hahn, who as early as 1926 suggested the possibility of the formation of isotopes of the 85th element during the beta decay of polonium.
For 19 years, from 1925 to 1943, at least half a dozen reports about the discovery of the ecaiod appeared in the periodical press. He was credited with certain chemical properties, assigned sonorous names: Helvetius (in honor of Switzerland), Anglo-Helvetius (in honor of England and Switzerland), Dakin (from the name ancient country Dacians in Central Europe), leptin (translated from Greek as “weak”, “shaky”, “destitute”), etc. However, the first reliable message about the discovery and identification of element No. 85 was made by physicists involved in the synthesis of new elements.
D. Corson, C. McKenzie, and E. Segre irradiated a bismuth target with alpha particles at the cyclotron at the University of California. The particle energy was 21 MeV, and nuclear reaction receiving item #85 was as follows:
209 83 Bi + 4 2 He → 211 85 At + 2 1 0 n.
The new synthetic element was named only after the war, in 1947. But even earlier, in 1943, it was proved that astatine isotopes are formed in all three rows of radioactive decay. Therefore, astatine is found in nature.
Astatine in nature
Astatine in nature was the first to be found by Austrian chemists B. Karlik and T. Bernert. Studying the radioactivity of the daughter products of radon, they found that a small part of radium-A (the so-called then, and still called, the 218 Po isotope) decays in two ways (the so-called radioactive fork):
In a freshly isolated sample of RaA, along with alpha particles generated by polonium-218, alpha particles with other characteristics were also detected. Just such particles could, according to theoretical estimates, emit nuclei of the isotope 218 85.
Later, short-lived isotopes 215 At, 216 At, and 217 At were discovered in other experiments. And in 1953, the American radiochemists E. Hyde and A. Ghiorso isolated the 219 At isotope from francium-223 by chemical means. This is the only case of chemical identification of an isotope of astatine from a naturally occurring isotope. It is much easier and more convenient to obtain astatine artificially.
discover, identify, find out
The above reaction of irradiation of bismus with alpha particles can also be used for the synthesis of other isotopes of astatine. It is enough to increase the energy of the bombarding particles to 30 MeV, as the reaction proceeds with the emission of three neutrons and astatine-210 is formed instead of astatine-211. The higher the energy of alpha particles, the more secondary neutrons are produced and the smaller, consequently, the mass number of the resulting isotope. As targets for irradiation, metallic bismuth or its oxide is used, which is deposited or deposited onto an aluminum or copper substrate.
Rice. 6.
Another method for the synthesis of astatine is to irradiate a gold target with accelerated carbon ions. In this case, in particular, the following reaction occurs:
197 79 Au + 12 6 C → 205 85 At + 4 1 0 n.
To isolate the resulting astatine from bismuth or gold targets, a rather high volatility of astatine is used - it is still a halogen! Distillation occurs in a stream of nitrogen or in vacuum when the target is heated to 300...600°C. Astatine condenses on the surface of a glass trap cooled with liquid nitrogen or dry ice.
Another way to obtain astatine is based on the reactions of fission of uranium or thorium nuclei when they are irradiated with alpha particles or high-energy protons. So, for example, when 1 g of metallic thorium is irradiated with protons with an energy of 680 MeV at the synchrocyclotron of the Joint Institute nuclear research in Dubna, about 20 microcuries (otherwise 3 10 13 atoms) of astatine are obtained. However, in this case it is much more difficult to isolate astatine from a complex mixture of elements. This difficult problem was solved by a group of radiochemists from Dubna headed by V.A. Khalkin.
Now 20 isotopes of astatine with mass numbers from 200 to 219 are already known. The longest-lived of them is the 210 At isotope (half-life 8.3 hours), and the shortest-lived is 214 At (2 10 -6 seconds).
Since astatine cannot be obtained in significant quantities, its physical and chemical properties are not fully understood, and physicochemical constants are most often calculated by analogy with more accessible neighbors in the periodic system. In particular, the melting and boiling points of astatine were calculated as 411 and 299°C, i.e. astatine, like iodine, should sublime more easily than melt.
All studies on the chemistry of astatine were carried out with ultra-small amounts of this element, on the order of 10–9 ... 10–13 g per liter of solvent. And it's not even that you can't get more concentrated solutions. If they could be obtained, it would be extremely difficult to work with them. The alpha radiation of astatine leads to the radiolysis of solutions, their strong heating and the formation of large amounts of by-products.
And yet, despite all these difficulties, despite the fact that the number of astatine atoms in solution is comparable to accidental (though carefully avoided) pollution, some progress has been made in studying the chemical properties of astatine. It has been established that astatine can exist in six valence states– from 1 – to 7+. In this, it manifests itself as a typical analogue of iodine. Like iodine, it dissolves well in most organic solvents, but it acquires a positive electric charge more easily than iodine.
The properties of a number of interhalogen compounds of astatine, for example, AtBr, AtI, CsAtI 2 , have been obtained and studied.
An attempt with suitable means
The first attempts to apply astatine in practice were made as early as 1940, immediately after obtaining this element. A group of employees of the University of California found that astatine, like iodine, is selectively concentrated in the thyroid gland. Experiments have shown that the use of 211 At for the treatment of thyroid diseases is more beneficial than radioactive 131 I.
Astatine-211 emits only alpha rays - very energetic at short distances, but not able to go far. As a result, they act only on the thyroid gland, without affecting the neighboring - parathyroid. The radiobiological effect of astatine alpha particles on the thyroid gland is 2.8 times stronger than that of beta particles emitted by iodine-131. This suggests that astatine is very promising as a therapeutic agent in the treatment of the thyroid gland. A reliable means of removing astatine from the body has also been found. The rhodanide ion blocks the accumulation of astatine in the thyroid gland, forming a strong complex with it. So element number 85 can no longer be called practically useless.
a brief description of
ASTAT (lat. Astatium) is one of the most important radioactive chemical elements in nature. It refers to VII group periodic system of Mendeleev. The atomic number is 85.
Astatine has no stable isotopes. radioactive isotopes astatine discovered by this time about 20, they are all very unstable. The longest-lived 210 At has a half-life T 1/2 of 8.3 hours. It is for this reason that the earth's surface layer (1.6 km), as shown by calculations, contains 69 mg of astatine-218. This is very little.
Discovery history
The discovery of astatine, like many other elements of the periodic system, was accidental. Long time repeated attempts by scientists different countries open element number 85 with all kinds of chemical and by physical means in natural objects were unsuccessful.
Only comparatively recently, in 1940, E. Segre, T. Corson, and W. McKenzie obtained the first 211 At isotope at Berkeley (USA) by bombarding bismuth with a particles accelerated in a cyclotron.
Astatine got its name from the Greek astatos, which means unstable. However, such a short shock name, like halogens, came relatively recently, and earlier it was called astatium, or astatine.
Only after artificial production astatine in 1940, it was found that 215 At, 216 At, 218 At and 219 At - 4 of its isotopes are formed in very unlikely branches of the three natural series of radioactive decay of uranium and thorium (5 * 10 -5 - 0.02%).
Properties
Physical Properties
How the pure metal astatine possesses unique property- sublimates in molecular form from aqueous solutions, none of the known elements has such an ability.
Astatine evaporates easily normal conditions, as well as in vacuum. It is also well adsorbed on metals - Ag, Au, Pt.
It is thanks to these properties that it is possible to isolate astatine from the products of bismuth irradiation. This is achieved by their vacuum distillation with the absorption of astatine by silver or platinum (up to 85%).
Chemical properties.
According to its chemical properties, astatine is close to both iodine and polonium. Thus, the chemical properties of astatine are very interesting and peculiar, since it simultaneously exhibits the properties of a metal and a non-metal (halogen). This is due to the position of astatine in Mendeleev's periodic system. On the one hand, it belongs to the group of halogens, and at the same time, it is the heaviest of them, acquiring “metallic” properties.
Astatine is precipitated by hydrogen sulfide even from strongly acidic solutions, as well as typical metals, is displaced by zinc from sulfate solutions. It is deposited on the cathode during electrolysis.
Astatine, like chlorine, gives insoluble astatine silver AgAt with silver; like iodine, it is oxidized to a 5-valent state (salt AgAtO 3 is similar to AgJO 3), but the main difference between astatine and iodine is radioactivity. The presence of astatine is determined by the characteristic a-radiation.
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