Americium

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Americium is a synthetic chemical element with the symbol Am and atomic number 95. It is a transuranic member of the actinide series, in the periodic table located under the lanthanide elements of europium, and thus by analogy named after America.

Americium was first produced in 1944 by group Glenn T. Seaborg of Berkeley, California, at the Metallurgical Laboratory of the University of Chicago, part of the Manhattan Project. Although it is the third element in the transuranic series, it is found fourth, after the heavier curium. The discovery was kept secret and was only released to the public in November 1945. Most of the americium produced by uranium or plutonium are bombarded with neutrons in nuclear reactors - a tonne of spent nuclear fuel contains about 100 grams of americium. It is widely used in commercial ionization smoke room detectors, as well as sources of neutrons and industrial gauges. Some unusual applications, such as nuclear batteries or fuel for spacecraft with nuclear propulsion, have been proposed for the <242m Am

Americium is a relatively soft radioactive metal with a silvery appearance. The common isotope is ²⁴¹ Am and ²⁴³ Am. In chemical compounds, americium usually assumes oxidation state 3, especially in solution. Several other oxidation states are known, ranging from 2 to 7 and can be identified by their characteristic optical absorption spectrum. The crystalline lattice of the solid americium and its compounds contains a small intrinsic radiogenic defect, because the self-irradiated metamictization with alpha particles, which accumulates with time; this may cause the drift of some material properties over time, more noticeable in older samples.

Video Americium

Histori

Although the americium was probably produced in previous nuclear experiments, it was first synthesized, isolated and identified in the late fall of 1944, at the University of California, Berkeley, by Glenn T. Seaborg, Leon O. Morgan, Ralph A. James, and Albert Ghiorso. They used a 60-inch cyclotron at the University of California, Berkeley. This element is chemically identified at the Metallurgical Laboratory (now Argonne National Laboratory) of the University of Chicago. After mild neptunium, heavier plaqueonium and curium, the americium is the fourth transuranium element found. At that time, the periodic table has been restructured by Seaborg to its current layout, containing the actinide row below the lanthanides. This causes the americium to be just below the left europium element; so the analogy is named after America: "The name americium (after America) and the symbol Am is suggested for the element on the basis of its position as the sixth member of the actinide rare earth series, analogous to europium, Eu, of the lanthanide series."

The new element is isolated from the oxide in a complex multi-step process. The first solution of plutonium-239 nitrate ( ²³⁹ PuNO ₃ ) was coated on a platinum foil about 0.5 cm ² , the solution was evaporated. and the residue is converted into plutonium dioxide (PuO ₂ ) by annealing. After cyclotron irradiation, the coating is dissolved with nitric acid, and then precipitated as hydroxide using a concentrated aqueous ammonia solution. The residue is dissolved in perchloric acid. Further separation is done by ion exchange, producing a specific curium isotope. The separation of the cerium and americium was so exhausting that the elements were originally referred to by the Berkeley group as pandemonium (from Greek for all demons or hell) and delirium (from Latin for madness ).

The second isotope ²⁴² Am is produced at the firing of a neutron from ²⁴¹ Am already made. After fast? -detik, ²⁴² Am converts to isotope isotope ²⁴² Cm (which has been found before). The half-life of this decay was initially determined at 17 hours, which approached the currently accepted value of 16.02 hours.

${\ displaystyle {\ ce {^ {241} _ {95} Am- & gt; [({\ ce {n}}, \ gamma)] {^ {242} _ {95} Am}}} \\ left ({\ ce {- & gt; [\ beta ^ {-}] [16.02 \ {\ ce {h}}] {^ {242} _ {96} Cm}}} \ right)}$

The invention of the americium and curium in 1944 was closely linked to the Manhattan Project; the results were confidential and not announced only in 1945. Seaborg leaked the 95th and 96th element synthesis on the US radio show for Kids Quiz Kids five days before the official presentation at the American Chemical Society meeting on November 11, 1945, when one of the listeners asked if there was a new transuranium element beside plutonium and neptunium had been found during the war. After the discovery of the americium isotope ²⁴¹ Am and ²⁴² Am, the patented production and compound only included Seaborg as its discoverer. The early american sample weighed several micrograms; they are barely visible and identified by their radioactivity. The first substantial amount of the 40-200 microgram metal americium was not prepared until 1951 by the reduction of americium (III) fluoride with barium metal in high vacuum at 1100 ° C.

Maps Americium

Genesis

The longest living and most common isotope ampotes, ²⁴¹ Am and ²⁴³ Am, have a beak of 432,2 and 7,370 years, respectively. Therefore, any primordial americium (the americium present on Earth during its formation) should have decomposed now.

There were americans concentrated in areas used for atmospheric nuclear weapons testing conducted between 1945 and 1980, as well as on nuclear incident sites, such as the Chernobyl disaster. For example, the analysis of debris at the site of testing of the first US hydrogen bomb, Ivy Mike, (1 November 1952, Enewetak Atoll), revealed the high concentrations of various actinides including the americium; but due to military secrecy, these results were not published until later, in 1956. Trinitite, the remaining glass remnants on the desert floor near Alamogordo, New Mexico, after a plutonium-based Trinity-based nuclear bomb test on July 16, 1945, contained traces amerisium-241. An increase in americium levels was also detected at the site of the Boeing B-52 US bomber crash, which carried four hydrogen bombs, in 1968 in Greenland.

In other regions, the mean surface radioactivity of the ground due to residual americium is only about 0.01 pixels/g (0.37 mBq/g). The atmospheric americium compound is insoluble in the general solvent and is largely attached to the soil particles. Soil analysis revealed about 1,900 times higher american concentrations in sandy soil particles than in water present in the pores of the soil; a higher ratio is measured in clay soil.

Americium is produced mostly artificially in small quantities, for research purposes. One ton of spent fuel contains about 100 grams of various american isotopes, most of the ²⁴¹ Am and ²⁴³ Am. Their prolonged radioactivity is undesirable to be discarded, and therefore the americium, along with other long-lived actinides, must be neutralized. The associated procedure may involve several steps, in which the americium was first separated and then converted by neutron bombing in a special reactor for short-lived nuclides. This procedure is known as nuclear transmutation, but it is still developed for americium. The transuranic elements from americium to fermium occur naturally in the natural nuclear fission reactor in Oklo, but no longer do so.

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Synthesis and extraction

Nucleotide isotope

Americium has been produced in small quantities in nuclear reactors for decades, and the isotope kilogram of ²⁴¹ Am and ²⁴³ Am has accumulated now. However, since it was first offered for sale in 1962, the price, about 1,500 USD per gram ²⁴¹ Am, remains virtually unchanged due to very complex separation procedures. The heavier isoters ²⁴³ Am produced in much smaller quantities; therefore more difficult to separate, resulting in higher ordering costs 100,000-160.000 USD/g.

Americium tidak disintesis langsung dari uranium - bahan reaktor yang paling umum - tetapi dari plutonium isotop ²³⁹ Pu. Yang terakhir perlu diproduksi pertama, sesuai dengan proses nuklir berikut:

${\ displaystyle {\ ce {^ {238} _ {92} U- & gt; [({\ ce {n}}, \ gamma)] {^ {239} _ {92} U} - & gt; [\ beta ^ {-}] [23.5 \ {\ ce {min}}] {^ {239} _ {93} Np} - & gt; [\ beta ^ {-}] [2.3565 \ {\ ce {d}}] {^ {239} _ {94} Pu}}}}  ÂÂ$

Pengambilan dua neutron oleh ²³⁹ Pu (reaksi yang disebut (n ,?)), diikuti oleh--decay, menghasilkan ²⁴¹ Am:

${\ displaystyle {\ ce {^ {239} _ {94} Pu- & gt; [2 ({\ ce {n}}, \ gamma)] {^ {241 } _ {94} Pu} - & gt; [\ beta ^ {-}] [14.35 \ {\ ce {yr}}] {^ {241} _ {95} Am}}}}  ÂÂ$

The plutonium present in spent fuel contains about 12% of the ²⁴¹ Pu. Because it spontaneously converts to ²⁴¹ Am, ²⁴¹ Pu can be extracted and can be used to produce further ²⁴¹ Am. However, the process is somewhat slow: half of the original amount of ²⁴¹ Pu decays to ²⁴¹ Am after about 15 years, and ²⁴¹ Am reaches maximum after 70 years.

Americium-242 has a half-life of only 16 hours, which converts further into the ²⁴³ Am, very inefficient. The last isotope produced as a substitute in a process where ²³⁹ Pu captures four neutrons under high neutron flux:

$Â\; ÂÂ\; Â\; Â\; ÂÂ\; Â\; Â\; Â\; Â{\displaystyle Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â{}_{94}{}^{239}\text{Pu}Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\stackrel{Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â4}{->}}$




















<                                   n                                 ,                 ? )                                                           Ã,                           94                                       243                                Pu                                    ->                               4,956                 Ã,                                   h                                                                               ?                                       -                                                                                            Am                                                        95               243                                         {\ displaystyle {\ ce {^ {239} _ {94} Pu- & gt; [4 ({\ ce {n}}, \ gamma)] \ _ {94} ^ {243} Pu- & gt; [\ beta ^ {-}] [4.956 \ {\ ce {h}}] {^ {243} _ {95} Am}}}}  Â

Generation metal

Most synthesis routines produce mixtures of different actinic acids in the form of oxides, from which the americium isotope can be separated. In typical procedures, spent fuel reactors (eg MOX fuels) are dissolved in nitric acid, and most uranium and plutonium are removed using PUREX type extraction ( P lutonium- UR anium EX traction) with tributyl phosphate in hydrocarbons. The remaining lanthanides and actinides are then separated from the aqueous residue (raffinate) by diamide-based extraction, to provide, after stripping, trivalent actinide mixtures and lanthanides. The Americium compound is then selectively extracted using chromatographic techniques and multi-step centrifugation with suitable reagents. A large amount of work has been done on the extraction of american solvents. For example, an EU-funded project 2003 codenamed "EUROPART" studied triazines and other compounds as potential extraction agents. A bis -tazazinyl bipyridine complex was proposed in 2009 because the reagent is highly selective for americium (and potassium). The separation of the americium from the very similar curium can be achieved by treating their hydroxide slurry in aqueous sodium bicarbonate with ozone, at high temperatures. Both Am and Cm are mostly present in solutions in valence state 3; while the curium remains unchanged, the americium oxidizes into a soluble Am (IV) complex that can be washed away.

Metallic amines are obtained by the reduction of the compounds. Americium (III) fluoride was first used for this purpose. Reactions were performed by using barium elements as reductors in water free and oxygen environments in tantalum and tungsten devices.

${\ displaystyle \ mathrm {2 \ AmF_ {3} \ \ 3 \ Ba \ \ longrightarrow \ 2 \ Am \ \ 3 \ BaF_ {2} Â Â$

Alternatifnya adalah pengurangan amerisium dioksida oleh lantanum metalik atau torium:

${\ displaystyle \ mathrm {3 \ AmO_ {2} \ \ 4 \ La \ \ longrightarrow \ 3 \ Am \ \ 2 \ La_ {2} O_ {3} }}  ÂÂ$

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Sifat fisik

In the periodic table, the americium is located to the right of plutonium, to the left of the curium, and under europium lanthanides, with which there are many similarities in physical and chemical properties. Americium is a very radioactive element. When it is freshly prepared, it has a silvery white metallic luster, but then slowly tarnish in the air. With a density of 12 g/cm ³ , americium less solid than curium (13.52 g/cm ³ ) and plutonium (19.8 g//sup>); but has a higher density than europium (5.264 g/cm ³ ) - largely because of its higher atomic mass. Americium is relatively soft and easy to be modified and has much lower modulus than the previous actinides: Th, Pa, U, Np and Pu. Melting point 1173 Â ° C is significantly higher than plutonium (639 ° C) and europium (826 ° C), but lower than kurium (1340 ° C).

At ambient conditions, americium is present at the most stable? shapes that have hexagonal crystal symmetry, and group spaces P6 ₃ /mmc with lattice parameters a Ã, = 346.8Ã, pm and c Ã, = 1124 pm, and four atoms per unit cell. Crystals consist of double-hexagonal packing with ABAC and isotypic coating sequences with -lanthanum and some actinides such as? -curium. The american crystal structure changes with pressure and temperature. When compressed at room temperature up to 5 GPa ,? -Am turned into? modification, which has a face-centered cube symmetry ( fcc ), group space Fm 3 m and lattice constants a Ã, = 489Ã, pm. This fcc structure is equivalent to the closest packaging to the ABC sequence. After further compression into 23 GPa, americium transformed into orthorhombic? Structure similar to uranium? No further transition was observed up to 52 GPa, except for monoclinic phase performance at pressures between 10 and 15 GPa. There is no consistency on the status of this phase in the literature, which is also sometimes a list ,? and? phases such as I, II and III. That? -? transition accompanied by a 6% decrease in crystal volume; although the theory also predicts a significant volume change for? -? transition, not experimentally observed. Pressure from? -? transition decreases with increasing temperature, and when? -heam is heated at ambient pressure, at 770 ° C turns into a different fcc phase from? -Am, and at 1075 ° C converts to a body-centered cubic structure. The phase diagram of the americium temperature pressure is somewhat similar to lanthanum, praseodymium and neodymium.

Like many other actinides, self-destruction of the crystal lattice due to the radiation of alpha particles is intrinsic to americium. This is particularly noticeable at low temperatures, where the mobility of the resulting lattice defects is relatively low, by extending the X-ray diffraction peak. This effect makes it somewhat uncertain american temperatures and some of its properties, such as electrical resistivity. Thus for americium-241, resistivity at 4.2 K increases with time from about 2 Ã,ÂμOhmÃ, Â · cm to 10 Ã,ÂμOhmÃ, Â · cm after 40 hours, and saturates about 16 Ã,ÂμOhmÃ, Â · cm after 140 hours. This effect is less clear at room temperature, due to the destruction of the radiation defects; also heating the temperature of the sample room stored for long hours at low temperatures restores its resistivity. In the new sample, resistivity gradually increases with a temperature of about 2 Ã,ÂμOhmÃ, Â · cm in liquid helium up to 69 Ã,ÂμOhmÃ, Â · cm at room temperature; This behavior is similar to that of neptunium, uranium, thorium and protactinium, but is different from plutonium and curium which shows a rapid increase of up to 60 ° K followed by saturation. The ambient temperature values ​​for americium are lower than neptunium, plutonium and curium, but higher than uranium, thorium and protactinium.

Americium is paramagnetic over a wide temperature range, from liquid helium, to room temperature and above. This behavior is very different from the neighboring curium which shows the antiferromagnetic transition at 52 ° C. The coefficient of americium thermal expansion is slightly anisotropic and amounts to (7,5 Â ± < 0,2) ÃÆ' - 10 ^-6 Ã,/Ã, Â ° C along the shorter axis a and (6.2 Â ± 0.4) ÃÆ' - 10 ^-6 Ã,/Ã, Â ° C for the longer hexagonal axis c . The enthalpy of the dissolution of the americium metal in hydrochloric acid under standard conditions is -620,6 Ã, Â ± 1,3Ã, kJ/mol , from which the standard enthalpy changes the formation (? < Sub> f H Ã, Â °) Am ³ aqueous -621,2 Â ± 2.0 kJ/mol . The standard potential of Am ³ /Am ⁰ is -2.08 Ã, Â ± 0.01Ã, V .

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Chemical properties

Americium readily reacts with oxygen and dissolves well in acids. The most common oxidation state for americium is 3, where the americium compound is somewhat stable against oxidation and reduction. In this sense, the americium is chemically similar to most lanthanides. The trivalent americanum forms insoluble fluoride, oxalate, iodate, hydroxide, phosphate and other salts. Another oxidation state has been observed between 2 and 7, which is the widest range among actinide elements. Their color in aqueous solution varies as follows: Am ³ (Am colorless to yellow-reddish), Am ⁴ (yellow-reddish), Am ^V O
₂ ; (yellow), Am ^VI O ² < br> ₂ (brown) and Am ^{VII 5 -} ₆ (dark green). All oxidation has the characteristics of its optical absorption spectrum, with some sharp peaks in the visible and mid-infrared regions, and the position and intensity of this peak can be converted to a concentration of the corresponding oxidation state. For example, Am (III) has two sharp peaks at 504 and 811 nm, Am (V) at 514 and 715 nm, and Am (VI) at 666 and 992 nm.

The Americium compound with oxidation state 4 and higher is a strong oxidizing agent, comparable in strength to ion permanganate ( MnO ^- ₄ . While the Am ⁴ ion is unstable in the solution and easily converted to Am ³ , the oxidation state 4 occurs well in solids, such as americium dioxide (Amo ₂ ) and americium (IV) fluoride (Amf ₄ ).

All pentavalent and hexavalent americanum compounds are complex salts such as KAmO ₂ F ₂ , Li ₃ AmO ₄ and Li < AmO ₆ , AmO ₂ F ₂ . This high oxidation rate Am (IV), Am (V) and Am (VI) can be prepared from Am (III) by oxidation with ammonium persulfate in dilute nitric acid, with silver (I) oxide in perchloric acid, or with ozone or sodium persulfate in sodium carbonate solution. The pentavalent oxidation status of the americium was first observed in 1951. It is present in aqueous solutions in the form of Amo
₂ ion (acidic) or Amo ^-
₃ ion (alkaline) is however unstable and subject to several rapid disproportion reactions:

${\ displaystyle {\ ce {3AmO2 4H - & gt; 2AmO2 ^ 2 Am ^ 3 2H2O}}}$

${\ displaystyle {\ ce {2Am ^ {V} - & gt; {Am ^ {VI}} Am ^ {IV}}}}$

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Chemical compounds

Oxygen compound

Three americic oxides are known, with oxidation levels of 2 (Amo), 3 (Am ₂ O ₃ ) and 4 (AmO ₂ ). Americium (II) oxide is prepared in minutes and has not been specified in detail. Americium (III) oxide is a red-brown solids with a melting point of 2205 ° C. Americium (IV) oxide is a major form of solid americide used in almost all of its applications. Like most other actinide dioxide, it is a black solid with a cubic crystal structure (fluorite).

The oxalate americium (III), a dry vacuum at room temperature, has a chemical formula Am ₂ (C ₂ O ₄ ) ₃ Ã, Â · 7H ₂ O. After heating in a vacuum, it loses water at 240 ° C and begins decomposing to Amo ₂ at 300 ° C , the decomposition is completed at about 470 ° C. The starting oxidate is soluble in nitric acid with a maximum solubility of 0.25 g/L.

Halide

The americium halide is known for the oxidation numbers 2, 3 and 4, where 3 is most stable, especially in solution.

The reduction of Am (III) compounds with sodium amalgam yields Am (II) ammonium halides AmCl ₂ , AmBr ₂ and Ami ₂ . They are very sensitive to oxygen and oxidized in water, releasing hydrogen and converting back to the Am (III) state. Specific lattice constants are:

• Orthorhombic AmCl ₂ : a = 896,3 Â ± 0.8Ã, pm , b = 757.3 Â ± 0.8Ã, pm and c = 453.2 Ã, Â ± 0.6 Ã, pm
• Tetragonal AmBr ₂ : a = 1 159 .2 Ã, Â ± 0.4Ã , Pm and c = 712.1 Ã, Â ± 0.3Ã, pm . They can also be prepared by reacting metallic americium with mercury halide HgX ₂ , where X = Cl, Br or I:

                 400                   -     ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ...      Â Â <Â> <500>                                        ?              Â       ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,                     <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<      ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,     ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,                    Â®      Â  ÂÂÂÂÂÂÂÂÂ,                        ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ...            AmX                             2        ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,                                        ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,          ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ/                                    Â   <Â> Hg                                 {\ displaystyle {\ ce {{Am} {\ underset {mercury \ halide} {HgX2}} - & gt; [{} \ atop 400-500 ^ {\ circ} {\ ce {C}}] {AmX2} {Hg}}}}  Â

Americium (III) fluoride (AmF ₃ ) tidak larut dengan baik dan mengendap pada reaksi ion Am ³ dan fluoride dalam larutan asam lemah:

${\ displaystyle {\ ce {Am ^ 3 3F ^ - - & gt; AmF3 (v)}}}  ÂÂ$

Tetravalen amerisium (IV) fluorida (Amf ₄ ) diperoleh dengan mereaksikan padatan (III) fluoride padat dengan fluor molekuler: