|Name , symbol , atomic number||Americium, Am, 95|
|Group , period , block||Ac , 7 , f|
|Appearance||silvery white metal|
|Atomic mass||243.061375 u|
|Atomic radius||184 pm|
|Van der Waals radius||228.5 pm|
|Electron configuration||[ Rn ] 5 f 7 7 s 2|
|1. Ionization energy||5.97381 (25) eV ≈ 576.38 kJ / mol|
|2. Ionization energy||11.7 (4) eV ≈ 1 130 kJ / mol|
|3. Ionization energy||21st.7 (4) eV ≈ 2 090 kJ / mol|
|4. Ionization energy||36.8 (4) eV ≈ 3 550 kJ / mol|
|5. Ionization energy||50.0 (1.9) eV ≈ 4 820 kJ / mol|
|density||13.67 g / cm 3|
|magnetism||paramagnetic ( Χ m = 7.1 · 10 −4 )|
|Melting point||1449 K (1176 ° C)|
|boiling point||2880 (2607 ° C)|
|Molar volume||1.778 10 −5 m 3 mol −1|
|Heat of evaporation||238.5 kJ / mol|
|Heat of fusion||14.4 kJ mol −1|
|Specific heat capacity||0.11 J kg −1 K −1|
|Electric conductivity||147.1 A V −1 m −1 at 293.15 K.|
|Thermal conductivity||10 W m −1 K −1 at 300 K|
|Oxidation states||+2, +3 , +4, +5, +6, (+7)|
|Normal potential||−2.070 V
(Am 3+ + 3 e - → Am)
|Electronegativity||1.3 ( Pauling scale )|
|For other isotopes see list of isotopes|
|Hazard and safety information|
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .
Americium ( chemical element with the element symbol Am and the atomic number 95. In the periodic table it is in the group of actinides ( 7th period , f-block ) and is also one of the transuranic elements . Besides europium, americium is the only element named after a continent . It is an easily deformable radioactive metal with a silvery-white appearance.) is a
There is no stable isotope of americium . On earth it occurs exclusively in an artificially produced form. The element was first created in late autumn 1944, but the discovery was not initially published. Curiously, its existence was revealed to the public on an American radio show for children by the explorer Glenn T. Seaborg , the show's guest.
Americium is formed in nuclear reactors , and one tonne of spent nuclear fuel contains an average of about 100 g of the element. It is used as a source of ionizing radiation, e.g. B. in fluorescence spectroscopy and in ionization smoke detectors . The americium isotope 241 Am was proposed because of its significantly longer half-life of 432.2 years compared to plutonium ( 238 Pu) for filling radionuclide batteries (RTG) for space probes , which would then provide electrical energy for operation for hundreds of years.
Americium was discovered in the late autumn of 1944 by Glenn T. Seaborg , Ralph A. James , Leon O. Morgan and Albert Ghiorso in the 60-inch cyclotron at the University of California at Berkeley and at the Metallurgical Laboratory of the University of Chicago (today: Argonne National Laboratory ) generated. After neptunium and plutonium , americium was the fourth transuranic uranium to be discovered since 1940; the curium , which is one atomic number higher , was the third produced in the summer of 1944. The name for the element was chosen based on the continent America - in analogy to europium , the rare earth metal that is exactly above americium in the periodic table : The name americium (after the Americas) and the symbol Am are 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.
As a rule, the oxides of the starting elements were used to create the new element . For this purpose, plutonium nitrate solution (with the isotope 239 Pu) was first applied to a platinum foil of about 0.5 cm 2 ; the solution then evaporated and the residue then calcined to oxide (PuO 2 ). After the bombardment in the cyclotron, the coating was dissolved using nitric acid, then again precipitated as the hydroxide using a concentrated aqueous ammonia solution; the residue was dissolved in perchloric acid. The further separation took place with ion exchangers . In their series of experiments, four different isotopes were generated in sequence: 241 Am, 242 Am, 239 Am and 238 Am.
They isolated 241 Am as the first isotope from a plutonium sample that was irradiated with neutrons . It breaks down into 237 Np when an α particle is emitted. The half-life of this α-decay was initially determined to be 510 ± 20 years; the value generally accepted today is 432.2 a.
- The times given are half-lives .
As a second isotope, 242 Am was found by renewed neutron bombardment of the previously generated 241 Am. The subsequent rapid β-decay results in 242 cm, the previously discovered curium . The half-life of this β-decay was initially determined to be 17 hours; the value determined to be valid today is 16.02 hours.
The discovery of the element was first made public in the American radio program Quiz Kids on November 11, 1945 by Glenn T. Seaborg, even before the actual announcement at a symposium of the American Chemical Society : One of the young listeners asked the show's guest, Seaborg whether new elements were discovered during the course of research into nuclear weapons during World War II . Seaborg answered yes and at the same time revealed the discovery of the next higher element, curium .
Americium ( 241 Am and 242 Am) and its production was later patented under the name "Element 95 and method of producing said element" , with only Glenn T. Seaborg being named as the inventor.
Nowadays, however, americium is bred as a by-product in nuclear power plants ; the americium isotope 241 Am arises as a decay product (e.g. in spent fuel rods ) from the plutonium isotope 241 Pu. One tonne of spent nuclear fuel contains, on average, about 100 g of different isotopes of americium. These are mainly the α-emitters 241 Am and 243 Am, which are undesirable in final disposal due to their relatively long half-lives and are therefore part of the transuranium waste . A reduction in long-term radio toxicity in nuclear repositories would be possible by separating long-lived isotopes from spent nuclear fuel. The partitioning & transmutation strategy is currently being investigated to eliminate the americium .
Extraction and presentation
Extraction of americium isotopes
Americium is produced in small quantities in nuclear reactors . Today it is available in quantities of a few kilograms. Due to the complex extraction from spent fuel rods , it has a very high price. Since its market launch in 1962, americium (IV) oxide with the isotope 241 Am has been said to have been priced at around 1500 US dollars per gram . The americium isotope 243 Am is produced in smaller quantities in the reactor from 241 Am and is therefore even more expensive at 160 US dollars per milligram of 243 Am.
Americium is inevitably produced via the plutonium isotope 239 Pu in nuclear reactors with a high 238 U content, as it is produced from this through neutron capture and two subsequent β decays (over 239 U and 239 Np).
- The times given are half-lives .
Thereafter, if nuclear fission does not occur, 239 Pu, along with other nuclides, is hatched through gradual neutron capture (n, γ) and subsequent β-decay 241 Am or 243 Am.
The plutonium, which can be obtained from spent fuel rods in power reactors, consists of about 12% of the isotope 241 Pu. This is why the spent fuel rods only reach their maximum content of 241 Am; 70 years after the breeding process has ended ; thereafter the content decreases again (more slowly than the increase).
The resulting 241 Am can be converted into 242 Am through further neutron capture in the reactor . In the case of light water reactors , the 241 Am should be 79% 242 Am and 10% 242 m Am:
For the breeding of 243 Am a quadruple neutron capture of the 239 Pu is necessary:
Depiction of elemental americiums
Metallic americium can be obtained from its compounds by reduction . First americium (III) fluoride was used for reduction. For this purpose, this is caused to react with elemental barium in reaction apparatuses made of tantalum and tungsten in an environment free of water and oxygen .
In the periodic table , americium with atomic number 95 is in the series of actinides, its predecessor is plutonium, the following element is curium. Its analogue in the series of lanthanides is europium .
Americium is a radioactive metal. Freshly made americium is a silver-white metal that slowly becomes matt at room temperature . It is easily deformable. Its melting point is 1176 ° C, the boiling point is 2607 ° C. The density is 13.67 g cm −3 . It occurs in two modifications.
The α-Am modification, which is stable under standard conditions, crystallizes in the hexagonal crystal system in the space group P 6 3 / mmc (space group no. 194) with the lattice parameters a = 346.8 pm and c = 1124 pm and four formula units per unit cell . The crystal structure consists of a double hexagonal close packing of spheres (i.e. hcp) with the layer sequence ABAC and is therefore isotypic to the structure of α-La .
At high pressure, α-Am changes to β-Am. The β-modification crystallizes in the cubic crystal system in the space group Fm 3 m (No. 225) with the lattice parameter a = 489 pm, which corresponds to a face-centered cubic lattice (fcc) or a cubic closest packing of spheres with the stacking sequence ABC.
The enthalpy of solution of americium metal in hydrochloric acid under standard conditions is −620.6 ± 1.3 kJ mol −1 . Based on this value, the standard enthalpy of formation (Δ f H 0 ) was calculated for the first time from Am 3+ (aq) to −621.2 ± 2.0 kJ mol −1 and the standard potential Am 3+ / Am 0 to −2, 08 ± 0.01 V.
The most stable oxidation state for americium is +3, the Am (III) compounds are very stable against oxidation and reduction. The americium is the first representative of the actinides, which behaves more like the lanthanoids than the d-block elements.
It can also be found in oxidation states +2 as well as +4, +5, +6 and +7. Depending on the oxidation number , the color of americium varies in aqueous solution as well as in solid compounds :
Am 3+ (yellow-pink), Am 4+ (yellow-red), Am V O 2 + (yellow), Am VI O 2 2+ (lemon yellow), Am VII O 6 5− (dark green).
The Am 4+ ions, which are not stable in aqueous solution , can only be produced from Am (III) using strong oxidizing agents. Two compounds of americium in the +4 oxidation state are known in solid form: americium (IV) oxide (AmO 2 ) and americium (IV) fluoride (AmF 4 ).
The pentavalent oxidation state was first observed in americium in 1951. In aqueous solution are primarily AmO 2 + ions (acid) or AmO 3 - ions (alkaline) before, but these are unstable and rapid disproportionation subject:
First of all, a disproportionation to the oxidation state +6 and +4 can be assumed:
The americium (VI) compounds are somewhat more stable than Am (IV) and Am (V). They can be prepared from Am (III) by oxidation with ammonium peroxodisulfate in dilute nitric acid. The typical pink tone disappears towards a strong yellow color. In addition, the oxidation with silver (I) oxide in perchloric acid can be achieved quantitatively. In sodium carbonate or sodium hydrogen carbonate solutions, oxidation with ozone or sodium peroxodisulfate is also possible.
A biological function of americium is not known. The use of immobilized bacterial cells to remove americium and other heavy metals from rivers has been proposed. Thus, enteric bacteria of the genus Citrobacter by the phosphatase activity in their cell wall precipitate certain Americiumnuklide from aqueous solution and bind as a metal-phosphate complex. The factors that influence the biosorption and bioaccumulation of americium by bacteria and fungi were also examined .
The isotope 242 m 1 Am has the highest thermal gap cross-section measured to date (10/2008) at around 5700 barn . This is accompanied by a small critical mass , which is why 242 m 1 Am was proposed as fissile material, for example to power spaceships with nuclear energy propulsion.
In principle, this isotope is also suitable for the construction of nuclear weapons . The critical mass of a pure 242 m 1 Am sphere is about 9-14 kg. The uncertainties of the available cross- sections currently do not allow a more precise statement. With the reflector, the critical mass is around 3–5 kg. In aqueous solution it is again greatly reduced. In this way, very compact warheads could be built. According to the state of public knowledge, no nuclear weapons from 242 m 1 Am have been built so far , which can be explained by the low availability and the high price.
For the same reasons, 242 m 1 Am is also not used as a nuclear fuel in nuclear reactors , although in principle it would be suitable for this in both thermal and fast reactors ., The two other more commonly available isotopes, 241 Am and 243 Am, can also be used in a fast reactor maintain a chain reaction. However, the critical masses are very high here. Unreflected, they are 57.6–75.6 kg at 241 Am and 209 kg at 243 Am so that their use does not result in any advantages over conventional fissile materials.
Correspondingly, americium is not legally assigned to nuclear fuels in accordance with Atomic Energy Act . However, there are proposals to construct very compact reactors with an americium inventory of just 20 g, which can be used in hospitals as a neutron source for neutron capture therapy.(1) of the
16 isotopes and 11 nuclear isomers with half-lives between fractions of microseconds and 7370 years are known of americium . There are two long-lived α-emitting isotopes 241 Am with 432.2 and 243 Am with 7370 years half-life. In addition, the core isomer 242 m 1 Am has a long half-life of 141 years. The remaining core isomers and isotopes have short half-lives of 0.64 µs at 245 m 1 Am to 50.8 hours at 240 Am.
241 Am is the most frequently incubated americium isotope and belongs to the Neptunium series . It decays with a half-life of 432.2 years with an α-decay to 237 Np. 241 Am emits the entire decay energy with the α-particle with a probability of 0.35%, but mostly emits one or more gamma quanta .
242 Am is short-lived and, with a half-life of 16.02 h, decays 82.7% through β-decay to 242 Cm and 17.3% through electron capture to 242 Pu. The 242 cm decays to 238 Pu and this further to 234 U, which is on the uranium-radium series . The 242 Pu decays via the same chain of decay as 238 Pu. However, while 238 Pu joins the decay chain as a side arm of 234 U, 242 Pu is still before 238 U. 242 Pu decays through α decay into 238 U, the beginning of the natural uranium-radium series.
242 m 1 Am decays with a half-life of 141 years to 99.541% by internal conversion to 242 Am and to 0.459% by α-decay to 238 Np. This decays to 238 Pu and then further to 234 U, which is on the uranium-radium series .
With a half-life of 7,370 years, 243 Am is the longest-lived americium isotope. It is first converted into 239 Np by α radiation , which further decays to 239 Pu by β decay . The 239 Pu decays through α-radiation to uranium 235 U, the official beginning of the uranium actinium series .
The two most long-lived isotopes 241 Am and 243 Am are of particular interest for the use of americium . Usually it is used in the form of the oxide (AmO 2 ).
Ionization smoke detector
The α radiation from the 241 Am is used in ionization smoke detectors . It is preferred over 226 Ra because it emits comparatively little γ radiation . To do this, however, the activity against radium must be about five times as much. The decay series of 241 Am “ends” for the period of use almost immediately after its α decay at 237 Np , which has a half-life of around 2.144 million years.
Because of its significantly longer half-life compared to 238 Pu, 241 Am was proposed for filling radionuclide batteries (RTG) of space probes . Thanks to its half-life of 432.2 years, an RTG with a 241 Am filling could provide electrical energy to operate a space probe for hundreds of years - instead of just a few decades (as with a 238 Pu filling). It is expected to be used in the radionuclide batteries that ESA is considering developing and that could be completed in the 2020s.
241 Am pressed as oxide with beryllium represents a neutron source that is used, for example, for radiochemical investigations. For this purpose, the high cross-section of beryllium is used for (α, n) nuclear reactions, with americium serving as the producer of the α particles. The corresponding reaction equations are:
In addition to the frequently used 210 Po as an ionizer to remove unwanted electrostatic charge , 241 Am was also used. For this purpose z. B. the source is mounted on the head of a brush with which one slowly stroked the surfaces to be treated and thus could avoid re-contamination by electrostatically attracted dust particles.
Manufacture of other items
Americium is the starting material for the production of higher transuranic elements and also the transactinoids . From 242 On arises to 82.7% curium ( 242 Cm) and 17.3% plutonium ( 242 Pu). In the nuclear reactor the 244 Am is inevitably hatched in small quantities by neutron capture from 243 Am , which decays to the curium isotope 244 Cm through β-decay .
With its intense gamma radiation - spectral line at 60 keV is suitable 241 On well as a radiation source for X-ray fluorescence spectroscopy . This is also used for calibration of gamma spectrometers used in the low energy region, since the adjacent lines are relatively weak and formed as a single peak standing. In addition, the peak is only negligibly disturbed by the Compton continuum of higher-energy lines, since these also occur at most with an intensity that is at least three orders of magnitude lower.
Safety instructions and dangers
Classifications according to the CLP regulation are not available because they only include chemical hazard, which plays a completely subordinate role compared to the hazards based on radioactivity . There is only a chemical hazard at all if it concerns a relevant amount of substance.
Since americium only has radioactive isotopes, it and its compounds may only be handled in suitable laboratories under special precautions. Most common americium isotopes are alpha emitters, which is why incorporation must be avoided. The broad spectrum of daughter nuclides, which are usually also radioactive, represents a further risk that must be taken into account when choosing safety precautions. 241 Am emits large amounts of relatively soft gamma radiation during radioactive decay, which can be shielded well.
According to research by the researcher Arnulf Seidel from the Institute for Radiation Biology at the Karlsruhe Nuclear Research Center , americium (like plutonium ), when absorbed into the body, generates more bone tumors than the same dose of radium .
→ Category: Americium compound
Americium (III) oxide (Am 2 O 3 ) is a red-brown solid and has a melting point of 2205 ° C.
Americium (IV) oxide (AmO 2 ) is the main compound of this element. Almost all applications of this element are based on this connection. It occurs implicitly in nuclear reactors when irradiating uranium dioxide (UO 2 ) or plutonium dioxide (PuO 2 ) with neutrons. It is a black solid and crystallizes - like the other actinide (IV) oxides - in the cubic crystal system in the fluorite structure.
Halides are known for their oxidation states +2, +3 and +4. The most stable level +3 is known for all compounds from fluorine to iodine and is stable in aqueous solution.
Americium (IV) fluoride
Americium (III) fluoride
Americium (III) chloride
Americium (III) bromide
Americium (III) iodide
Americium (II) chloride
Americium (II) bromide
Americium (II) iodide
The tetravalent americium was also observed in the aqueous phase.
Americium (III) chloride (AmCl 3 ) forms pink hexagonal crystals. Its crystal structure is isotype with uranium (III) chloride . The melting point of the compound is 715 ° C. The hexahydrate (AmCl 3 · 6 H 2 O) has a monoclinic crystal structure.
Am (II) salts are accessible by reduction with Na amalgam from Am (III) compounds: the black halides AmCl 2 , AmBr 2 and AmI 2 . They are very sensitive to oxygen and oxidise to Am (III) compounds in water, releasing hydrogen.
Chalcogenides and pentelides
Silicides and borides
Americium monosilicide (AmSi) and americium "disilicid" (AmSi x with: 1.87 <x <2.0) were obtained by reducing americium (III) fluoride with elemental silicon in a vacuum at 1050 ° C (AmSi) and 1150– 1200 ° C (AmSi x ) shown. AmSi is a black mass, isomorphic with LaSi. AmSi x is a light silver compound with a tetragonal crystal lattice.
Borides of the compositions AmB 4 and AmB 6 are also known.
Analogous to uranocene , an organometallic compound in which uranium is complexed by two cyclooctatetraene ligands, the corresponding complexes of thorium , protactinium , neptunium, plutonium and also of americium, (η 8 -C 8 H 8 ) 2 Am, were prepared.
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- The hazards emanating from radioactivity do not belong to the properties to be classified according to the GHS labeling. With regard to other hazards, this element has either not yet been classified or a reliable and citable source has not yet been found.
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