Records in science and technology. Elements. Meaning of the word astatine The last one received
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“Rare chemical elements and their application” “Astat” Prepared by Yulia Borzenkova, student of class 11B, MBOU Secondary School No. 5, Novocherkassk
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Introduction Astatine is an element of the main subgroup of the seventh group, the sixth period of the periodic table chemical elements D.I. Mendeleev, with atomic number 85. Denoted by the symbol At (lat. Astatium). Radioactive. The heaviest element of the known halogens. The simple substance astatine under normal conditions is unstable crystals of black-blue color. The astatine molecule is apparently diatomic (formula At2). Astatine is a toxic substance. Inhaling it in very small quantities can cause severe irritation and inflammation of the respiratory tract, and large concentrations lead to severe poisoning.
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Physical properties Astatine is a solid substance of a beautiful blue-black color, appearance similar to iodine. It is characterized by a combination of the properties of non-metals (halogens) and metals (polonium, lead and others). Like iodine, astatine is highly soluble in organic solvents and is easily extracted by them. It is slightly less volatile than iodine, but can also sublimate easily. Melting point 302 °C, boiling point (sublimation) 337 °C.
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Chemical properties Astatine has a low vapor pressure, is slightly soluble in water, and is better soluble in organic solvents. Astatine in aqueous solution is reduced by sulfur dioxide SO2; like metals, it is precipitated even from strongly acidic solutions by 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 capable of oxidizing to the At(V) state, like iodine (for example, the salt AgAtO3 is identical in properties to AgIO3). Astatine reacts with bromine and iodine, resulting in the formation of interhalogen compounds - astatine iodide AtI and astatine bromide AtBr: Both of these compounds are dissolved 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 - sodium astatide). Like other halogens, astatine can replace hydrogen in the methane molecule to produce tetraastatmethane CAt4. In this case, first astatomethane CH3At is formed, then diastatmethane CH2At2 and astatine form CHAt3. In positive oxidation states, 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 his colleagues (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, K. 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 natural radioactive series In Russian terminology, the element was initially called “astatine”. The names "Helvetine" (in honor of Helvetia - ancient name Switzerland) and “leptin” (from the Greek “weak, shaky”). The name comes from the Greek word "astatos", which literally means "unstable". And the element fully corresponds to the name given to it: its life is short, its half-life is only 8.1 hours.
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Astatine in nature Astatine is the rarest element found in nature. The 1.6 km thick surface layer of the earth's crust 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 radioactive series 235U and 238U. The rate of their formation is constant and equal to the rate of their radioactive decay, therefore earth's crust contain 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. They are all radioactive. The most stable of them (from 207At to 211At) have a half-life of more than an hour (the most stable is 210At, T1/2 = 8.1 hours); however, three natural isotopes have half-lives 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 the astatine by coprecipitation, extraction, chromatography, or distillation. Melting point 302 °C, boiling point (sublimation) 337 °C.
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Astatine isotopes Mass number Isotope mass relative to 16O Half-life Form and energy of radiation, MeV 202 - 43 s CDz; α, 6.50 203 - 102 s CDz; α, 6.35 203 420 s CDz; α, 6.10 204 - 1500 s K-z 205 - 1500 s Kz; α, 5.90 206 - 0.108 days KDz 207 - 6480 s K-z (90%); α (10%), 5.75 208 - 0.262 s KDz 208 6120 s K-z (>99%), α (0.5%), 5.65 209 - 0.229 s K-z (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 s α, 7.79 217 07225 0.018 s α, 7.02 218 07638 1.5D2.0 s 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 back in 1940, immediately after obtaining this element. Group of employees University of California found that astatine, like iodine, is selectively concentrated in the thyroid gland. Experiments have shown that using 211At for the treatment of thyroid diseases is more beneficial than radioactive 131I. Thyroid
There are 94 chemical elements found in nature. To date, another 15 transuranium elements have been artificially obtained (elements from 95 to 109), the existence of 10 of them is indisputable.
The most common
Lithosphere. Oxygen (O), 46.60% by weight. Discovered in 1771 by Karl Scheele (Sweden).
Atmosphere. Nitrogen (N), 78.09% by volume, 75.52% by mass. Discovered in 1772 by Rutherford (Great Britain).
Universe. Hydrogen (H), 90% of the total substance. Discovered in 1776 by Henry Cavendish (Great Britain).
Rarest (out of 94)
Lithosphere. Astatine (At): 0.16 g in the earth's crust. Opened in 1940 by Corson (USA) and his employees. The naturally occurring isotope astatine 215 (215 At) (discovered in 1943 by B. Karlik and T. Bernert, Austria) exists in quantities of only 4.5 nanograms.
Atmosphere. Radon (Rn): only 2.4 kg (6·10 –20 volume of one part per 1 million). Opened in 1900 by Dorn (Germany). The concentration of this radioactive gas in areas of granite rock deposits is believed to have caused a number of cancers. The total mass of radon found in the earth’s crust, from which atmospheric gas reserves are replenished, is 160 tons.
The easiest
Gas. Hydrogen (H) has a density of 0.00008989 g/cm 3 at a temperature of 0°C and a pressure of 1 atm. Opened in 1776 by Cavendish (Great Britain).
Metal. Lithium (Li), with a density of 0.5334 g/cm 3, is the lightest of all solids. Discovered in 1817 by Arfvedson (Sweden).
Maximum Density
Osmium (Os), with a density of 22.59 g/cm 3, is the heaviest of all solids. Discovered in 1804 by Tennant (Great Britain).
The heaviest gas
It is radon (Rn), the density of which is 0.01005 g/cm 3 at 0°C. Opened in 1900 by Dorn (Germany).
Last received
Element 108, or unniloctium (Uno). This provisional name is given by the International Union of Pure and Applied Chemistry (IUPAC). Obtained in April 1984 by G. Münzenberg and coworkers (West Germany), who observed only 3 atoms of this element in the laboratory of the Society for Heavy Ion Research in Darmstadt. In June of the same year, a message appeared that this element was also obtained by Yu.Ts. Oganesyan and his staff at the United Institute nuclear research, Dubna, USSR.
A single unnilenium atom (Une) was obtained by bombarding bismuth with iron ions in the laboratory of the Heavy Ion Research Society, Darmstadt, West Germany, on August 29, 1982. It has the highest atomic number (element 109) and the highest atomic mass (266) . According to the most preliminary data, Soviet scientists observed the formation of an isotope of element 110 with an atomic mass of 272 (preliminary name - ununnilium (Uun)).
The cleanest
Helium-4 (4 He), obtained in April 1978 by P.V. McLintock of Lancaster University, USA, has less than 2 parts of impurities per 10 15 parts of volume.
The hardest
Carbon (C). In its allotropic form, diamond has a Knoop hardness of 8400. Known since prehistoric times.
Dearest
Californian (Cf) was sold in 1970 at a price of $10 per microgram. Opened in 1950 by Seaborg (USA) and employees.
The most flexible
Gold (Au). From 1 g you can draw a wire 2.4 km long. Known since 3000 BC.
Highest tensile strength
Boron (B) – 5.7 GPa. Discovered in 1808 by Gay-Lussac and Thénard (France) and H. Davy (Great Britain).
Melting/boiling point
Lowest. Among non-metals, helium-4 (4He) has the lowest melting point -272.375°C at a pressure of 24.985 atm and the lowest boiling point -268.928°C. Helium was discovered in 1868 by Lockyer (Great Britain) and Jansen (France). Monatomic hydrogen (H) must be an incompressible superfluid gas. Among metals, the corresponding parameters for mercury (Hg) are –38.836°C (melting point) and 356.661°C (boiling point).
The tallest. Among non-metals, the highest melting point and boiling point is carbon (C), known since prehistoric times: 530°C and 3870°C. However, it seems controversial that graphite is stable at high temperatures. Transitioning from a solid to a vapor state at 3720°C, graphite can be obtained as a liquid at a pressure of 100 atm and a temperature of 4730°C. Among metals, the corresponding parameters for tungsten (W) are 3420°C (melting point) and 5860°C (boiling point). Opened in 1783 by H.H. and F. d'Eluyarami (Spain).
Isotopes
The largest number of isotopes (36 each) is found in xenon (Xe), discovered in 1898 by Ramsay and Travers (Great Britain), and in cesium (Cs), discovered in 1860 by Bunsen and Kirchhoff (Germany). Hydrogen (H) has the smallest amount (3: protium, deuterium and tritium), discovered in 1776 by Cavendish (Great Britain).
The most stable. Tellurium-128 (128 Te), according to double beta decay, has a half-life of 1.5 10 24 years. Tellurium (Te) was discovered in 1782 by Müller von Reichenstein (Austria). The isotope 128 Te was first discovered in its natural state in 1924 by F. Aston (Great Britain). Data on its superstability were again confirmed in 1968 by studies by E. Alexander Jr., B. Srinivasan and O. Manuel (USA). The alpha decay record belongs to samarium-148 (148 Sm) – 8·10 15 years. The beta decay record belongs to the cadmium isotope 113 (113 Cd) – 9·10 15 years. Both isotopes were discovered in their natural state by F. Aston, respectively, in 1933 and 1924. The radioactivity of 148 Sm was discovered by T. Wilkins and A. Dempster (USA) in 1938, and the radioactivity of 113 Cd was discovered in 1961 by D. Watt and R. Glover (Great Britain).
The most unstable. The lifetime of lithium-5 (5 Li) is limited to 4.4 10 –22 s. The isotope was first discovered by E. Titterton (Australia) and T. Brinkley (Great Britain) in 1950.
Liquid series
Given the difference between melting point and boiling point, the element with the shortest liquid range is the noble gas neon (Ne) - just 2.542 degrees (-248.594°C to -246.052°C), while the longest liquid range (3453 degrees) characteristic of the radioactive transuranic element neptunium (Np) (from 637°C to 4090°C). However, if we take into account the true series of liquids - from the melting point to the critical point - then the element helium (He) has the shortest period - only 5.195 degrees (from absolute zero to -268.928 ° C), and the longest - 10200 degrees - for tungsten (from 3420°C to 13,620°C).
The most poisonous
Among non-radioactive substances, the most stringent restrictions are set for beryllium (Be) - the maximum permissible concentration (MAC) of this element in the air is only 2 μg/m3. Among the radioactive isotopes existing in nature or produced by nuclear installations, the most stringent limits on the content in the air are set for thorium-228 (228 Th), which was first discovered by Otto Hahn (Germany) in 1905 (2.4 10 –16 g/m 3), and in terms of content in water – for radium-228 (228 Ra), discovered by O. Gan in 1907 (1.1·10 –13 g/l). From an environmental point of view, they have significant half-lives (i.e. over 6 months).
Guinness Book of Records, 1998
Discovery history:
Predicted (as “eka-iodine”) by D.I. Mendeleev in 1898. “... upon the discovery of a halogen X with an atomic weight greater than iodine, it will still form KX, KXO3, etc., that its hydrogen compound HX will be gaseous, a very weak acid, that the atomic weight will be ... 215”
Astatine was first obtained artificially in 1940 by D. Corson, K. R. Mackenzie and E. Segre (University of California at Berkeley). To synthesize the 211 At isotope, they irradiated bismuth with alpha particles. In 1943-1946, astatine isotopes were discovered as part of natural radioactive series.
The name Astatium is derived from the Greek. words ( astatoz) meaning "unstable".
Receipt:
Short-lived astatine radionuclides (215 At, 218 At and 219 At) are formed during the radioactive decay of 235 U and 238 U, this is due to the constant presence of traces of astatine in nature (~ 1 g). Basically, astatine isotopes are obtained by irradiation of metallic bismuth or thorium. a-high energy particles followed by separation of astatine by coprecipitation, extraction, chromatography or distillation. The mass number of the most stable known isotope is 210.
Physical properties:
Due to its strong radioactivity, it cannot be obtained in macroscopic quantities sufficient for an in-depth study of its properties. According to calculations, the simple substance astatine under normal conditions is unstable crystals of a dark blue color, consisting not of At 2 molecules, but of individual atoms. Melting point is about 230-240°C, boiling point (sublimation) - 309°C.
Chemical properties:
In terms of chemical properties, astatine is close to both iodine (shows properties of halogens) and polonium (properties of a metal).
Astatine in aqueous solution is reduced by sulfur dioxide; like metals, it is precipitated even from strongly acidic solutions by hydrogen sulfide, and is displaced from sulfate solutions by zinc.
Like all halogens (except fluorine), astatine forms an insoluble salt, AgAt (silver astatide). It is capable of oxidizing to the At(V) state, like iodine (for example, the salt AgAtO 3 is identical in properties to AgIO 3). Astatine reacts with bromine and iodine, resulting in the formation of interhalogen compounds - astatine iodide AtI and astatine bromide AtBr.
When an aqueous solution of astatine is exposed to hydrogen at the moment of reaction, gaseous hydrogen astatine HAt is formed, a substance that is extremely unstable.
Application:
The instability of astatine makes the use of its compounds problematic, however, the possibility of using various isotopes of this element to combat cancer has been studied. See also: Astatine // Wikipedia. . Update date: 05/02/2018. URL: https://ru.wikipedia.org/?oldid=92423599 (access date: 08/02/2018).
Discovery of elements and origin of their names.
What is astatine, why is it interesting and is it worth studying? After reading our article, you will learn a lot of interesting things about this peculiar chemical element, the history of its discovery and where it found application.
By arranging the chemical elements in increasing order of their atomic weights, the Russian chemist Dmitri Ivanovich Mendeleev discovered that in this natural series repeated periodically at regular intervals chemical elements with similar chemical properties. This is how the periodic law of D.I. was discovered. Mendeleev. At that time, science knew nothing about the structure of the atom. Therefore, as the basis for the classification of chemical elements, D.I. Mendeleev took size atomic mass And element properties.
Simpler meaning periodic law DI. Mendeleev can be expressed as follows:the properties of elements change smoothly and equally as their atomic weight increases, and then these changes are repeated periodically. Later, when science discovered the structure of the nucleus, the concept of “atomic weight” » replaced by the concept of “nuclear charge”.
So, according to the periodic law, the properties of elements should change smoothly. But this did not always happen. Sometimes in the sequence of changing the properties of elements was missing some link. In this case, Mendeleev left gaps in the table that had to be filled by newly discovered elements with the corresponding chemical characteristics. That is, with the help of his law, Mendeleev predicted the properties of elements that had not yet been discovered.
Astatine
Similarly, in 1898, Mendeleev predicted the existence The 85th element of the periodic table of chemical elements, which he named "eka-iodine". But the 85th element was obtained only in 1940 by American physicists D. Corson, K. McKenzie and E. Segre artificially. The new element was given a name astatine. In 1943, astatine was discovered in nature. Of all the elements found on Earth, astatine is the rarest. Astatine naturally contains only about 30 grams.
Translated from Greek, “astatos” means “unstable.” Indeed, astatine has a very short lifespan. Its half-life is only 8.3 hours, i.e. The astatine obtained in the morning decreases by half by the evening.
Chemical properties of astatine
Graphically periodic system D.I. The periodic table is represented by a two-dimensional table called the periodic table. Column number or group number in this table equal to the number electrons on the outer layer of an atom of a substance. The row number or period number is equal to the number of energy levels in the atom.
In the table of the periodic system, astatine is in group VII along with the halogens: fluorine, chlorine, bromine, iodine. The chemical activity of halogens decreases from fluorine to iodine. If we look at these substances, we will see that fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid with some properties of metals. Astatine is the fifth most heavy element halogen groups.
In terms of its chemical properties, astatine is similar to iodine, but differs from it in many ways, since it has the properties of metals more than iodine. Unlike iodine, astatine - radioactive element. Astatine also shows similarities to polonium, its neighbor on the left in the periodic table.
Like all halogens, astatine produces the insoluble salt AgAt. But, like typical metals, astatine is precipitated by hydrogen sulfide even from strongly acidic solutions, is displaced by zinc from sulfuric acid solutions, and during electrolysis is deposited on the cathode.
Astatine is insoluble in water, but like iodine, it dissolves well in organic solvents. Easily evaporates in air and vacuum.
Astatine has the unique ability to sublimate in molecular form (transition from solid state directly into gaseous, bypassing liquid) from aqueous solutions. No known element has such an ability.
Practical application of astatine
Where is astatine used?
Scientists have found that astatine, like iodine, accumulates in the thyroid gland. But the potency of astatine is stronger than iodine. Astatine has many isotopes, but they all live for a very short time. The most promising isotope for the treatment of thyroid diseases is 211At. In addition, astatine can be excreted from the human body with the help of thiocyanate ions. Hence, harmful effects the isotope 211At on other organs will be minimal. This allows us to conclude that the use of astatine in medicine is very promising.
Astatine, the fifth halogen, is the least common element on our planet, unless, of course, you count the transuranium elements. A rough calculation shows that the entire earth's crust contains only about 30 g of astatine, and this estimate is the most optimistic. At element No. 85 stable isotopes no, but the longest living radioactive isotope has a half-life of 8.3 hours, i.e. of the astatine received in the morning, not even half remains by the evening.
Thus, the name astatine – and in Greek αστατος means “unstable” – aptly reflects the nature of this element. Why then might astatine be interesting and is it worth studying it? Worth it, because astatine (as well as promethium, technetium and francium) in in every sense The word was created by man, and the study of this element provides a lot of instructive information - primarily for understanding the patterns in the changes in the properties of the elements of the periodic system. Exhibiting metallic properties in some cases and non-metallic properties in others, astatine is one of the most unique elements.
Until 1962, in Russian chemical literature this element was called astatine, but now the name “astatine” has been assigned to it, and this is apparently correct: neither the Greek nor the Latin name of this element (astatium in Latin) has the suffix “in” "
Search for ekaiod
D.I. Mendeleev called the latter halogen not only ecaiodine, but also halogen X. He wrote in 1898: “We can, for example, say that upon the discovery of halogen X with an atomic weight greater than iodine, it will still form KX, KXO 3, etc., that its hydrogen compound will be a gaseous, very weak acid, that the entire atomic value will be ... about 215.”
In 1920, the 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 an intensive search for element No. 85 in natural objects began.
In making assumptions about the properties of the 85th element, chemists proceeded from its location in periodic table and from data on the properties of this element’s neighbors on 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 a solid substance that exhibits, although small degree, properties of metals. Ecaiodine is the heaviest halogen. Obviously, it should be even more metal-like than iodine, and, having many properties of halogens, it is somehow similar to its neighbor on the left - polonium... Together with other halogens, ecaiodine, apparently, should be found in the water of the seas and oceans , drilling wells. They tried to look for it, like iodine, in seaweed, brines, etc. The English chemist I. Friend tried to find modern astatine and francium in the waters of the Dead Sea, which, as was known, contained more than enough halogens and alkali metals. To extract ekaiodine from the chloride solution, silver chloride was precipitated; Friend believed that the sediment would carry with it traces of element 85. However, neither X-ray spectral analysis nor mass spectrometry gave a positive result.
In 1932, chemists Polytechnic Institute State of Alabama (USA), led by F. Allison, reported that they isolated a product from monazite sand that 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 the 85th element 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. The properties of alabamium diverged sharply from the predictions of the periodic law. In addition, by this time it had become clear that all elements heavier than bismuth did not have stable isotopes. If we assumed the stability of element No. 85, science would face 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 idea that element 85 could be a product of the radioactive decay of other elements became the starting point for another large group of researchers searching for ekaiodine. The first in this group should be named the famous German radiochemist Otto Hahn, who back in 1926 suggested the possibility of the formation of isotopes of the 85th element during the beta decay of polonium.
Over the 19 years from 1925 to 1943, at least half a dozen reports of the discovery of ecaiod appeared in periodicals. It was credited with certain chemical properties and given sonorous names: helvetium (in honor of Switzerland), anglohelvetium (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 report of the discovery and identification of element No. 85 was made by physicists engaged in the synthesis of new elements.
At the University of California cyclotron, D. Corson, K. McKenzie and E. Segre irradiated a bismuth target with alpha particles. The particle energy was 21 MeV, and the nuclear reaction to produce element No. 85 was as follows:
209 83 Bi + 4 2 He → 211 85 At + 2 1 0 n.
New synthetic element received its name only after the war, in 1947. But even earlier, in 1943, it was proven that astatine isotopes are formed in all three radioactive decay series. Therefore, astatine exists in nature.
Astatine in nature
The Austrian chemists B. Karlik and T. Bernert were the first to discover astatine in nature. Studying the radioactivity of radon daughter products, they discovered that a small part of radium-A (as the isotope 218 Po was called then, and is still called now) decays in two ways (the so-called radioactive fork):
In the freshly isolated RaA sample, 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 21885.
Later, in other experiments, short-lived isotopes 215 At, 216 At and 217 At were discovered. And in 1953, American radiochemists E. Hyde and A. Ghiorso chemically isolated the isotope 219 At from France-223. This is the only case of chemical identification of an astatine isotope from a naturally occurring isotope. It is much easier and more convenient to obtain astatine artificially.
Detect, highlight, find out
The above reaction of irradiating bismus with alpha particles can also be used to synthesize other isotopes of astatine. It is enough to increase the energy of the bombarding particles to 30 MeV, and the reaction will proceed with the emission of three neutrons and instead of astatine-211, astatine-210 will be formed. The higher the energy of alpha particles, the more secondary neutrons are formed and the lower, therefore, the mass number of the isotope formed. Metallic bismuth or its oxide is used as irradiation targets, which are fused or deposited onto an aluminum or copper substrate.
Rice. 6.
Another method for synthesizing astatine involves irradiating 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, the fairly high volatility of astatine is used - it is, after all, a halogen! Distillation occurs in a stream of nitrogen or in a vacuum when the target is heated to 300...600°C. Astatine condenses on the surface of a glass trap cooled liquid nitrogen or dry ice.
Another method for producing astatine is based on the reactions of fission of uranium or thorium nuclei when irradiated with alpha particles or high-energy protons. 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 for 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 are already known with mass numbers from 200 to 219. The longest-lived isotope is 210 At (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 incompletely studied, and physicochemical constants are most often calculated by analogy with its more accessible neighbors in the periodic table. In particular, the melting and boiling points of astatine were calculated - 411 and 299°C, i.e. Astatine, like iodine, should sublimate more easily than melt.
All studies on the chemistry of astatine were carried out with ultra-small quantities of this element, on the order of 10 –9 ...10 –13 g per liter of solvent. And the point is not even that it is impossible to obtain more concentrated solutions. Even if it were possible to obtain them, it would be extremely difficult to work with them. Alpha radiation from astatine leads to radiolysis of solutions, their strong heating and the formation of large quantities of by-products.
And yet, despite all these difficulties, despite the fact that the number of astatine atoms in solution is comparable to accidental (albeit carefully avoided) contamination, in the study chemical properties Astatine has achieved some success. 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 electrical 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.
Trying with suitable means
The first attempts to apply astatine in practice were made back in 1940, immediately after obtaining this element. A group at the University of California found that astatine, like iodine, is selectively concentrated in the thyroid gland. Experiments have shown that using 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 capable of traveling far. As a result, they act only on the thyroid gland, without affecting the neighboring one - the parathyroid gland. The radiobiological effect of astatine alpha particles on the thyroid gland is 2.8 times stronger than 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. Rodanide ion blocks the accumulation of astatine in the thyroid gland, forming a strong complex with it. So element No. 85 can no longer be called practically useless.