|Name , symbol , atomic number||Curium, Cm, 96|
|Group , period , block||Ac , 7 , f|
|Appearance||silvery white metal|
|Atomic mass||247,0703 u|
|Atomic radius||174.3 pm pm|
|Electron configuration||[ Rn ] 5 f 7 6 d 1 7 s 2|
|1. Ionization energy||5.99141 (25) eV ≈ 578.08 kJ / mol|
|2. Ionization energy||12.4 (4) eV ≈ 1 200kJ / mol|
|3. Ionization energy||20th.1 (4) eV ≈ 1 940 kJ / mol|
|4. Ionization energy||37.7 (4) eV ≈ 3 640 kJ / mol|
|5. Ionization energy||51.0 (1.9 eV) ≈ 4 920 kJ / mol|
|density||13.51 g / cm 3|
|Melting point||1613 K (1340 ° C)|
|boiling point||3383 K (3110 ° C)|
|Molar volume||18.05 · 10 −6 m 3 · mol −1|
|Thermal conductivity||10 W m −1 K −1 at 300 K|
|Oxidation states||(+2), +3 , +4|
|Normal potential||−2.06 V
(Cm 3+ + 3 e - → Cm)
|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 .
Curium is an artificially produced chemical element with the element symbol Cm and the ordinal number 96. In the periodic table it is in the group of actinides ( 7th period , f-block ) and is one of the transuranic elements . Curium was named after the researchers Marie Curie and Pierre Curie .
Curium was first produced from the lighter element plutonium in the summer of 1944 ; the discovery was initially not published. It was only in an American radio show for children that the explorer Glenn T. Seaborg, as a guest on the show, revealed the existence of the public by answering a young listener's question as to whether new elements had been discovered.
Curium is a powerful α emitter ; it is occasionally used in radionuclide batteries due to the very large amount of heat generated during decay . It is also used to generate 238 Pu for low-gamma radionuclide batteries, for example in cardiac pacemakers . The element can also be used as a starting material for the production of higher transuranic elements and transactinides . It also serves as an α-radiation source in X-ray spectrometers, with which u. a. the Mars rovers Sojourner , Spirit and Opportunity chemically analyze rocks on the surface of the planet Mars . The lander Philae of the space probe Rosetta was supposed to use it to examine the surface of comet 67P / Churyumov-Gerasimenko .
Curium was discovered in the summer of 1944 by Glenn T. Seaborg and his collaborators Ralph A. James and Albert Ghiorso . In their series of experiments, they used a 60-inch cyclotron at the University of California at Berkeley . After neptunium and plutonium , it was the third transurane discovered since 1940 . Its production succeeded even before that of the element americium, which is one place lower in the atomic number .
The oxides of the original elements were mostly 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 was then evaporated and the residue was then calcined to form oxide (PuO 2 ). After bombardment in the cyclotron, the coating was dissolved using nitric acid and then precipitated again as the hydroxide using a concentrated aqueous ammonia solution ; the residue was dissolved in perchloric acid. The further separation took place with ion exchangers . Two different isotopes were produced in this series of experiments: 242 cm and 240 cm.
The identification was made unequivocally based on the characteristic energy of the α-particle emitted during the decay. The half-life of this α-decay was determined to be 150 days for the first time (162.8 d).
The second, more short-lived isotope 240 cm, which is also formed by bombarding 239 Pu with α-particles, was only discovered later in March 1945:
The half-life of the subsequent α-decay was determined for the first time to be 26.7 days (27 d).
Due to the ongoing Second World War , the discovery of the new element was initially not published. The public only found out about its existence in an extremely curious way: on the American radio program Quiz Kids on November 11, 1945, one of the young listeners asked Glenn Seaborg, who appeared as a guest on the program, whether there were any new ones during the Second World War in the course of research into nuclear weapons Elements were discovered. Seaborg replied in the affirmative, revealing the existence of the element at the same time as that of the next lower element, americium. This happened before the official announcement at a symposium of the American Chemical Society .
The discovery of curium ( 242 cm, 240 cm), its production, and that of its compounds were later patented under the name Element 96 and compositions thereof , the inventor being named Glenn T. Seaborg.
The name curium was chosen in analogy to gadolinium , the rare earth metal that is exactly above curium in the periodic table . The choice of name honored the couple Marie and Pierre Curie , whose scientific work in the study of radioactivity had been groundbreaking. It followed the naming of gadolinium , which was named after the famous explorer of the rare earths, Johan Gadolin : As the name for the element of atomic number 96 we should like to propose "curium", with symbol Cm. The evidence indicates that element 96 contains seven 5f electrons and is thus analogous to the element gadolinium with its seven 4f electrons in the regular rare earth series. On this basis element 96 is named after the Curies in a manner analogous to the naming of gadolinium, in which the chemist Gadolin was honored.
The first measurable amount of curium was produced in 1947 in the form of hydroxide by Louis B. Werner and Isadore Perlman . This was 40 μg of 242 cm, which was created by neutron bombardment of 241 Am. In its elementary form, it was only produced in 1951 by reducing curium (III) fluoride with barium .
The longest-lived isotope, 247 cm, has a half-life of 15.6 million years. For this reason, all of the primordial curium that the earth contained when it was formed has now disintegrated. Curium is artificially produced in small quantities for research purposes. It also occurs in small amounts in spent nuclear fuel.
Most of the curium found in the environment comes from atmospheric nuclear weapons tests up until 1980. Locally, there are higher occurrences due to nuclear accidents and other nuclear weapon tests. However , curium hardly contributes to the natural background of the earth.
In addition to the first discovery of Einsteinium and Fermium in the remains of the first American hydrogen bomb, Ivy Mike , on November 1, 1952 on the Eniwetok Atoll, in addition to plutonium and americium , isotopes of curium, berkelium and californium were found: especially the isotopes 245 cm and 246 cm, in smaller amounts 247 cm and 248 cm, in tracks 249 cm. For reasons of military secrecy, the results were not published until later in 1956.
Extraction and presentation
Extraction of curium isotopes
Curium occurs in small amounts in nuclear reactors . Today it is only available worldwide in quantities of a few kilograms, which is why its very high price of about 160 US dollars per microgram 244 cm or 248 cm is based. In nuclear reactors, it is formed from 238 U in a series of nuclear reactions. An important step here is the (n, γ) - or neutron capture reaction, in which the generated excited daughter nuclide changes to the ground state by emitting a γ quantum . The free neutrons required for this are created by fission of other nuclei in the reactor. In this nuclear chemical process, the plutonium isotope 239 Pu is initially formed by an (n, γ) reaction followed by two β - decays . In breeder reactors this process is used to incubate new fissile material.
- The times given are half-lives .
Two further (n, γ) reactions with subsequent β - decay produce the americium isotope 241 Am. After a further (n, γ) -reaction with the following β-decay, this gives 242 Cm.
For research purposes, curium can be extracted more efficiently from plutonium, which is available on a large scale from spent nuclear fuel. For this purpose, this is irradiated with a neutron source that has a high neutron flux. The possible neutron fluxes are many times higher than in a nuclear reactor, so that a different reaction path than the one shown above predominates here. From 239 Pu, four successive (n, γ) reactions form 243 Pu, which decays to the americium isotope 243 Am by β-decay with a half-life of 4.96 hours . The 244 Am formed by a further (n, γ) -reaction finally decays to 244 Cm again by β-decay with a half-life of 10.1 hours .
This reaction also takes place in nuclear fuel in nuclear power plants, so that 244 cm is also obtained in the reprocessing of spent nuclear fuel and can be obtained in this way.
The next heavier isotopes are formed from 244 cm through further (n, γ) reactions in the reactor in smaller and smaller quantities. The isotopes 247 cm and 248 cm are particularly popular in research because of their long half-lives. However, the formation of 250 Cm in this way is very unlikely, since 249 Cm has only a short half-life and so further neutron captures are unlikely in the short time. However, this isotope is accessible from the α-decay of 254 Cf. The problem here, however, is that 254 Cf decays mainly through spontaneous fission and only to a small extent through α decay. 249 Cm disintegrates through β - decay to Berkelium 249 Bk.
- (n, γ) reactions for the nucleon numbers A = 244–248, rarely also for A = 249 and 250.
However, curium produced by cascades of (n, γ) reactions and β decays always consists of a mixture of different isotopes. A separation is therefore associated with considerable effort.
Because of its long half-life, 248 cm is preferred for research purposes. The most efficient method for representing this isotope is given by the α-decay of Californium 252 Cf, which is accessible in larger quantities due to its long half-life. 248 Cm obtained in this way has an isotope purity of 97%. About 35–50 mg of 248 Cm are currently received in this way per year.
The interesting only for isotope research pure 245 Cm may be made of the α-decay of californium 249 Cf are obtained which of β in very small quantities as a daughter nuclide - -decay of Berkeliumisotops 249 Bk can be obtained.
Representation of elementary Curiums
Metallic curium can be obtained from its compounds by reduction . First, curium (III) fluoride was used for reduction. For this purpose, this is caused to react with elemental barium or lithium in reaction apparatuses made of tantalum and tungsten in an environment free of water and oxygen .
In the periodic table , curium with atomic number 96 is in the series of actinides, its predecessor is americium, the following element is berkelium. Its analogue in the lanthanoid series is gadolinium .
Curium is a radioactive metal. This is hard and has a silvery-white appearance similar to gadolinium, its lanthanoid analogue. It is also very similar in its other physical and chemical properties. Its melting point of 1340 ° C is significantly higher than that of the previous transuranic elements neptunium (637 ° C), plutonium (639 ° C) and americium (1173 ° C). In comparison, gadolinium melts at 1312 ° C. The boiling point of curium is 3110 ° C.
There is only one known modification of Curium under standard conditions with α-Cm . This crystallizes in the hexagonal crystal system in the space group P 6 3 / mmc (space group no. 194) with the lattice parameters a = 365 pm and c = 1182 pm as well as four formula units per unit cell . The crystal structure consists of a double hexagonal close packing of spheres with the layer sequence ABAC and is therefore isotypic to the structure of α-La .
Above a pressure of 23 GPa , α-Cm changes to β-Cm. The β-modification crystallizes in the cubic crystal system in the space group Fm 3 m (No. 225) with the lattice parameter a = 493 pm, which corresponds to a face-centered cubic lattice (fcc) or a cubic closest packing of spheres with the stacking sequence ABC.
The fluorescence of excited Cm (III) ions is long enough to be used for time-resolved laser fluorescence spectroscopy . The long fluorescence can be traced back to the large energy gap between the basic term 8 S 7/2 and the first excited state 6 D 7/2 . This allows the targeted detection of curium compounds while largely suppressing interfering short-lived fluorescence processes by other metal ions and organic substances.
The most stable oxidation state for curium is +3. Occasionally it can also be found in the +4 oxidation state. Its chemical behavior is very similar to americium and many lanthanoids . In dilute aqueous solutions the Cm 3+ ion is colorless, the Cm 4+ ion is pale yellow. In more concentrated solutions, however, the Cm 3+ ion is also pale yellow.
Curium ions belong to the hard Lewis acids , which is why they form the most stable complexes with hard bases. The complex bond has only a very small covalent component and is mainly based on ionic interaction. Curium differs in its complexation behavior from the previously known actinides such as thorium and uranium and is also very similar to the lanthanoids. In complexes it prefers a nine- fold coordination with a triple-capped trigonal- prismatic geometry.
The odd curium isotopes, in particular 243 cm, 245 cm and 247 cm, are in principle also suitable as nuclear fuels in a thermal nuclear reactor due to the large gap cross-sections . In general, all isotopes between 242 cm and 248 cm as well as 250 cm can maintain a chain reaction, even if in some cases only with rapid cleavage. Any mixture of the isotopes mentioned could be used as fuel in a fast reactor . The advantage then lies in the fact that no isotope separation would have to be carried out in the extraction from spent nuclear fuel , but only a chemical separation of the curium from the other substances.
The table below gives the critical masses for a pure spherical geometry without moderator and reflector:
|242 cm||371 kg||40.1 cm|
|243 cm||7.34-10 kg||10-11 cm|
|244 cm||(13.5) -30 kg||(12.4) -16 cm|
|245 cm||9.41-12.3 kg||11-12 cm|
|246 cm||39-70.1 kg||18-21 cm|
|247 cm||6.94-7.06 kg||9.9 cm|
|248 cm||40.4 kg||19.2 cm|
|250 cm||23.5 kg||16.0 cm|
With the reflector, the critical masses of the odd-numbered isotopes are around 3–4 kg. In an aqueous solution with a reflector, the critical mass for 245 cm can be reduced to 59 g ( 243 cm: 155 g; 247 cm: 1.55 kg); Due to the uncertainties of the physical data relevant for the calculation, these values are only accurate to about 15%; accordingly, the information in different sources varies considerably. Due to the low availability and the high price, curium is not used as a nuclear fuel and is therefore not classified as such in Atomic Energy Act in Germany.(1) of the
The odd curium isotopes, here again in particular 245 cm and 247 cm, could be used for the construction of nuclear weapons as well as for reactor operation . However, bombs from 243 cm would require considerable maintenance due to the short half-life of the isotope. In addition, as an α-emitter, 243 cm would have to be glowing hot due to the energy released during radioactive decay, which would make the construction of a bomb very difficult. Since the critical masses are sometimes very small, comparatively small bombs can be constructed in this way. So far, however, no activities of this kind have become public, which can also be attributed to the low availability.
Curium only has radionuclides and no stable isotopes . A total of 20 isotopes and 7 core isomers of the element between 233 cm and 252 cm are known. The longest half-lives have 247 cm with 15.6 million years and 248 cm with 348,000 years. In addition, the isotopes 245 Cm with 8500, 250 Cm with 8300 and 246 Cm with 4760 years have long half-lives. 250 cm is a special feature, since its radioactive decay consists for the most part (about 86%) in spontaneous fission .
The curium isotopes most frequently used technically are 242 cm with a half-life of 162.8 days and 244 cm with a half-life of 18.1 years.
The cross-sections for induced fission by a thermal neutron are: for 242 cm about 5 b , 243 cm 620 b, 244 cm 1.1 b, 245 cm 2100 b, 246 cm 0.16 b, 247 cm 82 b, 248 cm 0 , 36 b. This corresponds to the rule according to which mostly the transurane nuclides with an odd number of neutrons are "thermally easily fissile".
Since the two most frequently incubated isotopes, 242 Cm and 244 Cm, only have short half-lives (162.8 days and 18.1 years) and alpha energies of about 6 MeV , it shows a much stronger activity than that in natural uranium -Radium decay series produced 226 Ra . Because of this radioactivity, it gives off very large amounts of heat; 244 cm emits 3 watts / g and 242 cm even 120 watts / g. These curium isotopes can, due to the extreme heat development, in the form of curium (III) oxide (Cm 2 O 3 ) in radionuclide batteries to supply electrical energy e.g. B. be used in space probes . For this purpose, the use of 244 cm was examined in particular . As an α emitter, it requires a much thinner shield than the beta emitter, but its spontaneous fission rate and thus the neutron and gamma radiation is higher than that of 238 Pu. Because of the thick shielding and strong neutron radiation required, as well as its shorter half-life (18.1 years), it was therefore subject to 238 Pu with a half-life of 87.7 years.
242 cm was also used to generate 238 Pu for radionuclide batteries in pacemakers. 238 Pu hatched in the reactor is always contaminated with 236 Pu due to the (n, 2n) reaction of 237 Np , in whose decay series the strong gamma emitter 208 Tl occurs. The same applies to 238 Pu obtained from uranium under deuteron bombardment. The other curium isotopes, which are usually produced in relevant quantities in the reactor, quickly lead in their decay series to long-lived isotopes, the radiation of which is then no longer relevant for the construction of pacemakers.
244 cm serves as the α-radiation source in the α-particle X-ray spectrometers ( APXS ) developed by the Max Planck Institute for Chemistry in Mainz , with which the Mars rovers Sojourner , Spirit and Opportunity chemically analyzed rocks on the planet Mars. The Philae lander of the Rosetta space probe is also equipped with an APXS to analyze the composition of the Churyumov-Gerasimenko comet .
In addition, the lunar probe Surveyor had 5-7 alpha spectrometers on board. However, these worked with 242 cm and measured the protons knocked out of the lunar soil by the α-particles and the α-particles thrown back.
Manufacture of other items
Furthermore, curium is the starting material for the production of higher transuranic elements and transactinides . For example, bombarding 248 cm with oxygen ( 18 O) or magnesium cores ( 26 Mg) leads to the elements seaborgium 265 Sg or hassium 269 Hs and 270 Hs.
Classifications according to the CLP regulation are not available because they only include chemical hazards that play a completely subordinate role compared to the hazards based on radioactivity . The latter also only applies if the amount of substance involved is relevant.
Since only radioactive isotopes exist of curium, it and its compounds may only be handled in suitable laboratories with special precautions. Most common curium isotopes are α-emitters, which is why incorporation must be avoided. A large part of the isotopes also decays to a certain extent with spontaneous splitting . 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.
Effect in the body
If curium is ingested with food, most of it is excreted within a few days and only 0.05% is absorbed into the bloodstream. From here about 45% is deposited in the liver , another 45% is incorporated into the bone substance . The remaining 10% are eliminated. In the bone, curium accumulates, particularly on the inside of the interfaces with the bone marrow . The further spread into the cortex takes place only slowly.
When inhaled , Curium is absorbed much better into the body, which is why this type of incorporation represents the greatest risk when working with Curium. The maximum permissible total exposure of the human body to 244 cm (in soluble form) is 0.3 µ Ci .
In animal experiments with rats, an increased incidence of bone tumors was observed after an intravenous injection of 242 cm and 244 cm , the occurrence of which is considered to be the main risk in the ingestion of curium by humans. Inhalation of the isotopes resulted in lung and liver cancer .
Nuclear reactor waste problem
In nuclear reactors that are used economically (i.e. with a long retention period of the fuel), curium isotopes are physically unavoidable through (n, γ) nuclear reactions with subsequent β - decay (see also above under the extraction of curium isotopes). One ton of spent nuclear fuel contains an average of about 20 g of different curium isotopes. This also includes the α-emitters with the mass numbers 245-248, which are undesirable in the final disposal due to their relatively long half-lives and therefore count as transurane 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 pursued to eliminate the Curium . A three-stage process is planned in which the nuclear fuel is to be separated, processed in groups and disposed of. As part of this process, separated curium isotopes are to be converted into short-lived nuclides by neutron bombardment in special reactors. The development of this process is the subject of current research, although the process maturity has not yet been reached at this time.
Connections and reactions
The black curium (IV) oxide can be represented directly from the elements. For this purpose, metallic curium is annealed in air or in an oxygen atmosphere. The glowing of Curium salts is ideal for small quantities . Mostly curium (III) oxalate (Cm 2 (C 2 O 4 ) 3 ) or curium (III) nitrate (Cm (NO 3 ) 3 ) are used.
A number of ternary oxidic curium compounds of the type M (II) CmO 3 are also known.
Most of the Curium occurring in the wild (see section " Occurrence ") is present as Cm 2 O 3 and CmO 2 .
Curium (IV) fluoride
Curium (III) fluoride
Curium (III) chloride
Curium (III) bromide
Curium (III) iodide
The colorless curium (III) fluoride (CmF 3 ) can be obtained by adding fluoride ions to solutions containing Cm (III) . The tetravalent curium (IV) fluoride (CmF 4 ) is only accessible through the reaction of curium (III) fluoride with molecular fluorine :
The colorless curium (III) chloride (CmCl 3 ) can be produced by the reaction of curium (III) hydroxide (Cm (OH) 3 ) with anhydrous hydrogen chloride gas . This can be used to synthesize the other halides, curium (III) bromide (light green) and iodide (colorless). For this purpose, curium (III) chloride is reacted with the ammonium salt of the halide:
Chalcogenides and pentelides
The pentelids of the Curium of the CmX type have been shown for the elements nitrogen , phosphorus , arsenic and antimony . They can be produced by the reaction of either curium (III) hydride (CmH 3 ) or metallic curium with these elements at elevated temperatures.
Analogous to uranocene , an organometallic compound in which uranium is complexed by two cyclooctatetraene ligands, the corresponding complexes of thorium , protactinium , neptunium, plutonium and americium were represented. The MO scheme suggests that a corresponding compound (η 8 -C 8 H 8 ) 2 Cm, a curocene , can be synthesized, but this has not yet been achieved.
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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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