Opening history:
Predicted (as "eka-iodine") by D. I. Mendeleev in 1898. “... upon 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 a gaseous, very fragile acid, that the atomic weight will be ... 215”
Astatine was first obtained artificially in 1940 by D. Corson, C. 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 in the composition of natural radioactive series.
The name Astatium is derived from the Greek. the 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 irradiating metallic bismuth or thorium. a-particles of high energy, followed by separation of astatine by co-precipitation, 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 a deep study of its properties. According to calculations, the simple substance astatine under normal conditions is unstable dark blue crystals, consisting not of At 2 molecules, but of individual atoms. The melting point is about 230-240°C, the boiling point (sublimation) is 309°C.
Chemical properties:
In terms of chemical properties, astatine is close to both iodine (shows the properties of halogens) and polonium (metal properties).
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 sulfuric acid solutions by zinc.
Like all halogens (except fluorine), astatine forms an insoluble salt of AgAt (silver astatide). It is able to oxidize to the state At (V), like iodine (for example, AgAtO 3 salt is identical in properties to AgIO 3). Astatine reacts with bromine and iodine to form interhalogen compounds - astatine iodide AtI and astatine bromide AtBr.
When an aqueous solution of astatine is exposed to hydrogen, gaseous hydrogen astatide HAt is formed at the time of the reaction, the substance 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. . Date of update: 05/02/2018. URL: https://ru.wikipedia.org/?oldid=92423599 (date of access: 08/02/2018).
The discovery of the elements and the origin of their names.
What is astatine, why is it interesting and is it worth studying it? After reading our article, you will learn a lot of interesting things about this peculiar chemical element, about the history of its discovery and where it has found application.
Arranging the chemical elements in ascending 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. So the periodic law of D.I. Mendeleev. At that time, science knew nothing about the structure of the atom. Therefore, D.I. Mendeleev took atomic mass and element properties.
The meaning of D.I.'s periodic law is simpler. Mendeleev can be rendered as follows:the properties of the elements change smoothly and equally with an increase in their atomic weight, and then these changes are periodically repeated. 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 the elements should change smoothly. But this was not always the case. Sometimes in the sequence of changing the properties of elements was missing some link. In this case, Mendeleev left gaps in the table, which were 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 yet undiscovered elements.
Astatine
Similarly, in 1898, Mendeleev predicted the existence 85th element of the periodic table of chemical elements, which he called "eka-iodine". But the 85th element was obtained only in 1940 by American physicists D. Corson, C. Mackenzie and E. Segre by artificial means. The new element has been given a name. astatine. In 1943, astatine was discovered in nature. Of all the elements found on Earth, astatine is the rarest. In nature, astatine contains only about 30 grams.
Translated from the Greek "astatos" means "unstable". Indeed, astatine has a very short lifespan. Its half-life is only 8.3 hours, i.e. the astatine received in the morning by the evening is reduced by half.
Chemical properties of astatine
Graphically, the periodic system of D.I. The periodic table is displayed by a two-dimensional table, called the periodic table. The column number or group number in this table is equal to the number of electrons in the outer layer of an atom of the substance. The row number or period number is equal to the number of energy levels in the atom.
In the periodic table, astatine is in group VII along with halogens: fluorine, chlorine, bromine, iodine. The chemical activity of halogens decreases from fluorine to iodine. If we consider these substances, we will see that fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid substance with some properties of metals. Astatine is the fifth and heaviest element of the halogen group.
In terms of its chemical properties, astatine is similar to iodine, but in many respects it differs from it, since it has the properties of metals more than iodine. Unlike iodine, astatine is a radioactive element. Astatine also bears a resemblance to polonium, its neighbor on the left in the periodic table.
Like all halogens, astatine gives the insoluble AgAt salt. But, like typical metals, astatine is precipitated by hydrogen sulfide even from strongly acidic solutions, is displaced by zinc from sulfuric acid solutions, and is deposited on the cathode during electrolysis.
Astatine is insoluble in water, but, like iodine, dissolves well in organic solvents. Easily evaporates in air and vacuum.
Astatine has a unique ability to sublimate in molecular form (pass from a solid state immediately into a gaseous state, bypassing a liquid one) from aqueous solutions. None of the known elements have this ability.
Practical application of astatine
Where is astatine used?
Scientists have found that astatine, like iodine, accumulates in the thyroid gland. But in terms of strength, astatine is stronger than iodine. Astatine has many isotopes, but they all live for a very short time. The most promising for the treatment of thyroid diseases is the 211 At isotope. In addition, astatine can be excreted from the human body with the help of thiocyanate ions. Consequently, the harmful effects of the 211 At isotope on other organs will be minimal. This allows us to conclude that the use of astatine in medicine is very promising.
Astatium (Astatium), At (From Greek αστατος - unstable) - a radioactive chemical element of group VII of the periodic system of 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 amounts 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, the 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, on the one hand, exhibits 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) at room temperature, is soluble in organic solvents, and concentrates in the thyroid gland. As a pure metal, astatine behaves surprisingly: 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 strongly acidic 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 must be separated from large quantities of 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:
1 slide
Description of the slide:
"Rare chemical elements and their application" "Astat" Prepared by Julia Borzenkova Pupil of grade 11B MBOU secondary school No. 5 in Novocherkassk
2 slide
Description of the slide:
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 black-blue crystals. The astatine molecule appears to be diatomic (formula At2). Astatine is a poisonous substance. Inhalation of it in a very small amount can cause severe irritation and inflammation of the respiratory tract, and a large concentration leads to severe poisoning.
3 slide
Description of the slide:
Physical properties Astatine is a beautiful blue-black solid, similar in appearance 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.
4 slide
Description of the slide:
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.
5 slide
Description of the slide:
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. In positive oxidation states, astatine forms an oxygen-containing form, which is conventionally designated as Atτ+ (astatine-tau-plus).
6 slide
Description of the slide:
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" (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, the half-life is only 8.1 hours.
7 slide
Description of the slide:
Astatine in nature Astatine is the rarest element 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.
8 slide
Description of the slide:
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 of 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.
9 slide
Description of the slide:
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%)
10 slide
Description of the slide:
Application 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 211At for the treatment of thyroid diseases is more beneficial than radioactive 131I. Thyroid
a brief description of
ASTAT (lat. Astatium) is one of the most important radioactive chemical elements in nature. It belongs to the VII group of the periodic system of Mendeleev. The atomic number is 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 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. For a long time, repeated attempts by scientists from different countries to discover element No. 85 by various chemical and physical methods 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 the artificial production of 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
As a pure metal, astatine has a unique property - it sublimes in molecular form from aqueous solutions; none of the known elements has such an ability.
Astatine evaporates easily both under normal conditions and in a 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, like typical metals, it is displaced by zinc from sulfuric acid 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.
