Chinese name: mbth: Fermi element symbol: Fm atomic weight: 257 element type: Actinide element type: metal element discoverer: atomic number of jiaosuo: 100 Hazard: brief history of radioactive discovery, physical properties, chemical properties, mineral distribution, preparation method, nuclear reactor generation, and brief history of application discovery in synthesis field and nuclear explosion Author: jiaosuo time Americium was discovered in the fallout after the explosion of the first hydrogen bomb in 1952, and was named after Enrico Fermi, a nuclear physicist who won the Nobel Prize. Its chemical properties conform to the typical properties of heavier actinides, and tend to form +3 oxidation state, but it can also form +2 state. Due to the low yield, americium has no practical use outside the basic scientific research. Like other synthetic isotopes, americium is highly radioactive and toxic. Am was first discovered in the fallout of the first successfully detonated hydrogen bomb "Ivy Mike" 1952+065438+ 10/. After a preliminary examination of radioactive fallout, scientists discovered a new polonium isotope (24494Pu), which can only be formed by six neutrons absorbed by uranium -238 and then two beta transformations. At that time, it was generally believed that neutron absorption by heavy nuclei was a rare phenomenon, but the formation of 24494Pu meant that uranium nuclei might absorb more neutrons, thus producing heavier elements. Element 99 (raccoon) was soon found on the filter paper that had been in contact with the explosion cloud. (24494Pu was also found when the plane carrying filter paper flew over the settling cloud. ) 1952 12 In February, albert ghiorso and others identified the raccoon element at the University of California, Berkeley. They found the isotope Es (half-life is 20.5 days). This isotope was formed after the uranium -238 nucleus captured 15 neutrons, and then experienced seven beta decays: some U atoms can capture 16 or 17 neutrons. The discovery of americium (Z = 100) needs more research samples, because its yield is expected to be at least one order of magnitude less than that of raccoons. Therefore, the contaminated coral reef in Eniwetok Atoll, where nuclear tests were conducted, was sent to Lawrence Berkeley National Laboratory in California for treatment and analysis. Two months after the nuclear test, researchers separated some samples and found that it emitted high-energy alpha particles (7. 1 MeV) with a half-life of about 1 day. Such a short half-life means that it must come from the beta decay of an isotope, that is, the sample itself must be the isotope of the new element 100. Soon, the decay source was identified as Fm( t = 20.07(7) hours). Because it was during the Cold War, the discovery information of this new element and the new data about neutron capture were classified as secrets by the US military, and it was not released until 1955. However, Berkeley's team synthesized elements 99 and 100 by neutron impact on polonium -239, and published the research results in 1954. A note is attached to the report, pointing out that these contents have been studied before. The study of "Ivy Mike" nuclear bomb was declassified in 1955. Berkeley's team is worried that other research teams will find lighter americium isotopes by ion impact before their confidential research results are published. In fact, a team from the Nobel Institute of Physics in Stockholm, Sweden, discovered this element alone. They hit the 23892U target with oxygen-16 ions, and synthesized the isotope FM (t = 30min), and released this discovery in May 1954. However, it is generally believed that the Berkeley team discovered americium earlier, so the team has the right to name it. They decided to name it Fermion in memory of Enrico Fermi, the father of nuclear bullets. The name of americium commemorates the physical properties and relative atomic mass of Enrico Fermi: 257 americium electronegativity: 1.3 peripheral electron configuration: 5f 127s2 extranuclear electron configuration: 2,8,18,32,30,8 isotopes and radiation: FM-249. Sum of electron affinity energy of Fm+3: 0kJ mol-65438* Valence radius: 0 angstrom Chemical properties Am is a synthetic element with symbol Fm and atomic order 100, belonging to actinides. Americium is the heaviest element that can be produced by hitting a lighter element with neutrons, that is, it is the last element that can be produced in large quantities. People haven't made pure americium yet. There are 19 known isotopes in americium * *, among which Fm has the longest retention time and half-life of 100.5 days. Isotopes: At present, NUBASE 2003 lists the isotopes of americium in 19, with atomic weights ranging from 242 to 260, among which Fm has the longest survival time and a half-life of 100.5 days. The half-lives of Fm are 3 days, 5.3 hours, 25.4 hours, 3.2 hours, 20. 1 hour and 2.6 hours respectively. The half-lives of other isotopes range from 30 minutes to several milliseconds. Fm and Fm formed by neutron capture spontaneously split, and the half-life is only 370( 14) milliseconds. Fm and Fm are also extremely unstable and spontaneously split (half-lives are 1.5(3) seconds and 4 milliseconds, respectively). This means that neutron capture cannot be used to make nuclides with mass number higher than 257 unless it is produced in a nuclear explosion. Since Fm decays in α, and it does not undergo β transformation (which will form the next element: samarium), americium is the last element that can be produced by neutron capture. The chemical study of americium is carried out by tracer method in solution, and it has not been made into a solid compound so far. Generally speaking, americium is in Fm ionic state in the solution, with hydration number of 16.9 and acidity coefficient of 1.6× 10(p K a = 3.8). Fm will complex with various organic ligands with hard electron-donating atoms (such as oxygen), and the complex formed is generally more stable than the actinides before americium. It will also form complex ions with ligands such as chlorine and nitrogen, which are more stable than those formed by cesium or californium. It is considered that the complex bonds formed by heavier actinides are mainly ionic bonds: due to the high effective nuclear charge of americium, Fm ions are expected to be smaller than the an ions formed by previous actinides, which enables americium to form shorter and stronger chemical bonds with ligands. Fm can be easily reduced to Fm, for example, americium will precipitate with samarium dichloride. The electrode potential of americium is expected to be similar to that of ytterbium (Ⅲ) and ytterbium (Ⅱ), and the relative standard electrode potential is about ㄢ. 15 V, which is consistent with the theoretical calculation. The electrode potential between Fm and Fm is 3. 37( 10) V。 Toxicity: Although few people have been exposed to americium, the International Commission on Radiation Protection still makes recommendations on the annual radiation doses of the two most stable isotopes of americium. The intake dose limit of americium -253 is 10Bq( 1 Bq is equivalent to decay once per second), and the inhalation dose limit is10bq; Americium -257 is 10Bq and 4000 Bq respectively. Compounds At present, americium compounds are called Fm2O3 (americium trioxide), FmCl3 (americium trichloride) and FmF3 (americium trifluoride). Because all isotopes of americium have a short half-life, all primitive americium nuclides, that is, americium that may have existed when the earth was formed, still exist. Americium can also be produced by multi-neutron capture of actinides (uranium and thorium) in the earth's crust, but this possibility is extremely small. Therefore, almost all americium on the earth is produced in scientific laboratories, high-energy nuclear reactors or nuclear weapons tests, and it can only last for less than a few months after synthesis. Raccoons and americium were once naturally produced in Gapeng Oklo's natural nuclear reactor, but they are no longer formed. The preparation method bombards transuranic elements lighter than americium with particles lighter than bombarded atoms. Americium can also be prepared by neutron capture. The americium produced by nuclear reactor is produced by neutron hitting actinides in nuclear reactor. Americium -257 is the heaviest isotope produced by neutron capture energy, and its yield can reach nanogram at most (1× 10g). Americium is mainly produced in the 85 MW Qualcomm rate isotope reactor (HFIR) located in Oak Ridge National Laboratory, Tennessee, USA. The reactor is specially used for manufacturing ultracurium elements (Z >；; 96)。 Every time actinium is irradiated in the laboratory, tens of grams (/kloc-0 /×10g) of californium, several milligrams (/kloc-0 /×10g) of thulium and osmium, and several picograms (1 Or do experiments with a few nanograms (1× 10g) or a few micrograms (1× 10g) of americium. The amount of americium produced by a thermonuclear explosion of 200,000-200,000 tons is estimated to be only a few micrograms, but it is mixed with a large number of residual fragments. In the Hatch nuclear test conducted on July 6, 1969 in/kloc-0, 40 picograms of Fm was extracted from the debris of 10 kg. After americium is produced, it must be separated from other actinides and lanthanides produced by fission. Ion exchange chromatography is generally used, and a cation exchanger (e.g., Dowex 50 or TEVA) diluted in an ammonia solution of α -hydroxyisobutyrate is used. The smaller the positive ion is, the more stable the complex formed with the negative ion of α -hydroxyisobutyric acid is, so the layer is preferentially extracted in the elution column. Another method uses separate crystallization. Although Fm is the most stable americium isotope with a half-life of 100.5 days, Fm is used in most studies with a half-life of 20.07(7) hours. This is because the latter is the decay product of Es (half-life is 39.8( 12) days), and it is easy to separate. It is a long-term project to analyze the fallout of nuclear explosion100000 ton nuclear bomb "Ivy Mike", and its purpose is to study the production efficiency of transuranic elements in high-energy nuclear explosion. The reasons for using nuclear explosion are as follows: the transformation of uranium into transuranic elements requires multiple neutron capture, and the capture probability increases with the increase of neutron flux. Nuclear explosion is the strongest neutron source, which can produce 10 neutrons per square centimeter per microsecond (about 10 neutrons/(cm s)). In contrast, the neutron flux of Qualcomm rate isotope reactor is only 5× 10 neutron/(cm s). A laboratory was immediately set up at the explosion site of Eniwetok Atoll to make a preliminary analysis of radioactive fallout, because some isotopes may have decayed before being sent to the United States. After the nuclear explosion, the plane flew over the atoll with filter paper and immediately sent the samples back to the laboratory. At first, people hoped to find elements heavier than americium, but after a series of million-ton nuclear tests from 1954 to 1956, these elements were still not found. Because nuclear explosion in confined space may increase the possibility of producing heavy elements, the Nevada test base (now Nevada National Security Zone) conducted underground nuclear tests in the1960s and collected data. In addition to ordinary uranium, nuclear bombs also contain americium and a mixture of thorium and uranium, as well as a mixture of polonium and neptunium. Because the loaded heavy elements increase the fission rate and lead to heavier isotope loss, the yield of test results is less. Because atomic dust is distributed in molten vaporized rock 300-600 meters underground, drilling and sampling efficiency is very low at this depth, and it is also very difficult to extract separated products. Of the nine underground nuclear tests from 1962 to 1969, the last one was the largest and the output of transuranic elements was the highest. In the graph of the relationship between yield and atomic mass number (left), the yield of isotopes with lower mass and odd mass number is lower, so a zigzag curve is produced in the graph. This is because the isotopes of odd nuclei have higher fission rate. The biggest problem in the research is to collect atomic dust scattered everywhere after the explosion. Aircraft carrying filter paper only absorbed 4× 10 of the total amount, and the amount collected at Eniwetok Atoll only increased by two orders of magnitude. Sixty days after Hatch's nuclear test, only 10 was extracted from 500 kg of rocks. This 500 kg rock contains only 30 times more transuranic elements than the 0.4 kg rock obtained 7 days after the explosion. This proves that the amount of transuranic elements is not proportional to the weight of collected rocks. In order to speed up the sample collection, people drilled several shafts at the source of the explosion before the nuclear test, so that the explosion would bring enough samples from the center to the surface through the shafts to facilitate sampling. This method was tried in "Anacostia" and "Kennebec" nuclear tests, and hundreds of kilograms of materials were immediately provided for research, but the concentration of actinides was three times less than that of samples obtained by drilling. Although this method can effectively help to study isotopes with short retention time, it cannot improve the yield of actinides as a whole. Although this series of nuclear tests did not produce any new elements (except plutonium and americium) and the amount of transuranic elements obtained was not ideal, the total amount of rare heavy isotopes produced was still more than that synthesized in the previous laboratory. The thermal neutron-induced fission of Fm was studied with 6× 10 Fm atoms obtained from Hatch nuclear test, and a new americium isotope Fm was produced. There are also a large number of rare Cm isotopes collected, which are difficult to produce from Cm: the half-life of Cm (64 minutes) is too short compared with the radiation required by the reactor for several months, but it is very long for the nuclear explosion period. The application domain has no practical use.