Description of the presentation by individual slides:
1 slide
Slide description:
“Rare chemical elements and their application” “Astat” Prepared by Yulia Borzenkova, student of class 11B, MBOU Secondary School No. 5, Novocherkassk
2 slide
Slide description:
Introduction Astatine is an element of the main subgroup of the seventh group, the sixth period of the periodic system of chemical elements of D.I. Mendeleev, with atomic number 85. It is designated 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.
3 slide
Slide description:
Physical properties Astatine is a solid substance of a beautiful blue-black color, similar in appearance 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.
4 slide
Slide description:
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.
5 slide
Slide description:
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).
6 slide
Slide description:
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, isotopes of astatine were discovered as part of natural radioactive series. In Russian terminology, the element was initially called “astatine”. The names “helvetin” (in honor of Helvetia, the ancient name of Switzerland) and “leptin” (from the Greek “weak, shaky”) were also proposed. 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.
7 slide
Slide description:
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 the earth's crust contains a relatively constant equilibrium amount of astatine isotopes.
8 slide
Slide description:
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.
Slide 9
Slide description:
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%)
10 slide
Slide description:
Application 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 211At for the treatment of thyroid diseases is more beneficial than radioactive 131I. Thyroid
Discovery history:
Predicted (as “eka-iodine”) by D.I. Mendeleev in 1898. “... when a halogen X is discovered 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.
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. Element No. 85 has no stable isotopes, and the longest-lived 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? It’s worth it, because astatine (as well as promethium, technetium and francium) in the full sense of the word was created by man, and the study of this element provides a lot of instructive information - primarily for understanding the patterns in 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, and 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 the 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 that 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 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 at the Alabama Polytechnic Institute (USA), led by F. Allison, reported that they had 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 of the ancient country of the Dacians in Central Europe), leptin (translated from Greek as “weak”, “shaky” ", "dispossessed"), 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.
The 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 with 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 (though carefully avoided) contamination, 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 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.
a brief description of
ASTAT (lat. Astatium) is one of the most important radioactive chemical elements in nature. It belongs to group VII of the periodic system of Mendeleev. Atomic number - 85.
Astatine has no stable isotopes. There are about 20 radioactive isotopes of astatine discovered so far, all of them are 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 calculations have shown, contains 69 mg of astatine-218. This is very little.
History of discovery
The discovery of astatine, like many other elements of the periodic table, was accidental. For a long time, repeated attempts by scientists from different countries to discover element No. 85 using all kinds of chemical and physical methods in natural objects were unsuccessful.
Only relatively recently, in 1940, E. Segre, T. Corson and W. MacKenzie obtained the first isotope 211 At in Berkeley (USA), bombarding bismuth with a particles accelerated at a cyclotron.
Astatine gets its name from the Greek astatos, which means unstable. However, such a short shock name, like halogens, came relatively recently, and previously it was called astatium, or astatine.
Only after the artificial production of astatine in 1940 was it established that 215 At, 216 At, 218 At and 219 At - 4 of its isotopes are formed in very unlikely branches of three natural radioactive decay series of uranium and thorium (5 * 10 -5 - 0.02 %).
Properties
Physical properties
As a pure metal, astatine has a unique property - it sublimes in molecular form from aqueous solutions; no other known element has such an ability.
Astatine easily evaporates both under normal conditions and in a vacuum. It also adsorbs well on metals - Ag, Au, Pt.
It is thanks to these properties that it is possible to isolate astatine from the irradiation products of bismuth. This is achieved by vacuum distillation with the absorption of astatine by silver or platinum (up to 85%).
Chemical properties.
In terms of its chemical properties, astatine is close to both iodine and polonium. Thus, the chemical properties of astatine are very interesting and unique, since it simultaneously exhibits the properties of a metal and a non-metal (halogen). This is explained by the position of astatine in the periodic table of Mendeleev. 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, like typical metals, and is replaced by zinc from sulfate solutions. It is deposited on the cathode during electrolysis.
Astatine, like chlorine, produces insoluble astatine silver AgAt with silver; like iodine, it is oxidized to a 5-valent state (the 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.
Astat), At, non-metallic radioactive chemical element, atomic number 85, atomic mass 210.1. General description
Has isotopes with at. V. 202-219, of which At 211 (7.5 hours) and At 210 (8.3 hours) have the longest half-lives. A. has not been found in nature; it was first obtained artificially by bombarding Bismuth with α-particles. A. for chemistry properties similar to halogens and metals.
2. History
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, isotopes of astatine were discovered as part of natural radioactive elements.
3. Origin of the name
Melting point 302? C, boiling point (sublimation) 337? C.
6.2. Chemical properties
The properties of astatine are very similar to iodine: it is distilled, extracted with carbon tetrachloride CCl 4 from aqueous solutions, reduced by zinc or sulfur dioxide to astatide ion At -:
,which with silver ions forms the insoluble silver astatide AgAt. The latter is quantitatively precipitated with silver iodide as a carrier. Astatate Ion AtO - 3 is formed by the oxidation of astatide ion with periodic acid H 5 IO 6 or cerium Ce (IV):
The formalized recording of this equation corresponds to the condition of electrical neutrality. In fact, Ce(IV) ions exist in the form of hydrated ions 4, forming hydrogen ions and, with the exception of very acidic solutions (pH ~ 1), then undergo hydrolysis and polymerization. AtO 3 ions are quantitatively precipitated with water-insoluble Pb (IO 3) 2.
