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4 décembre 2011 7 04 /12 /décembre /2011 03:47

The actinide or actinoid (IUPAC nomenclature) series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.

The actinide series derives its name from the group 3 element actinium.HP Pavilion DV4-1199EA Battery

All but one of the actinides are f-block elements, corresponding to the filling of the 5f electron shell; lawrencium, a d-block element, is also generally considered an actinide. In comparison with the lanthanides, also mostly f-block elements, the actinides show much more variable valence.HP Pavilion DV4-1199EE Battery

Of the actinides, thorium and uranium occur naturally in substantial, primordial, quantities and small amounts of persisting natural plutonium have also been identified. The radioactive decay of uranium produces transient amounts of protactinium, and atoms of neptunium are occasionally produced from transmutation reactions in uranium ores.HP Pavilion DV4-1199EF Battery

The other actinides are purelysynthetic elements. Nuclear weapons tests have released at least six of these synthetic actinides into the environment; analysis of debris from a 1952 hydrogen bomb explosion showed the presence of americium, curium, berkelium, californium,einsteinium and fermium.HP Pavilion DV4-1199EO Battery

All actinides are radioactive and release energy upon radioactive decay; naturally occurring uranium and thorium, and synthetically produced plutonium are the most abundant actinides on Earth. These are used in nuclear reactors and nuclear weapons. Uranium and thorium also have diverse current or historical uses, and americium is used in the ionization chambers of most modernsmoke detectors.HP Pavilion DV4-1199EP Battery

In presentations of the periodic table, the lanthanides and the actinides are customarily shown as two additional rows below the main body of the table, with placeholders or else a selected single element of each series (either lanthanum or lutetium, and either actinium or lawrencium, respectively) HP Pavilion DV4-1199ER Battery

shown in a single cell of the main table, between barium and hafnium, and radium and rutherfordium, respectively. This convention is entirely a matter of aesthetics and formatting practicality; a rarely used wide-formatted periodic table inserts the lanthanide and actinide series in their proper places, as parts of the table's sixth and seventh rows (periods).HP Pavilion DV4-1199ES Battery

Discovery, isolation and synthesis

Like the lanthanides, the actinides form a family of elements with similar properties. Within the actinides, there are two overlapping groups: transuranium elements, which follow uranium in the periodic table—and transplutonium elements, which follow plutonium.HP Pavilion DV4-1199ET Battery

Compared to the lanthanides, which (except for promethium) are found in nature in appreciable quantities, most actinides are rare. The most abundant, or easy to synthesize actinides are uranium and thorium, followed by plutonium, americium, actinium, protactinium and neptunium.HP Pavilion DV4-1200 CTO Battery

The existence of transuranium elements was suggested by Enrico Fermi based on his experiments in 1934. However, even though four actinides were known by that time, it was not yet understood that they formed a family similar to lanthanides. HP Pavilion DV4-1200 Battery

The prevailing view that dominated early research into transuranics was that they were regular elements in the 7th period, with thorium, protactinium and uranium corresponding to 6th-period hafnium, tantalum and tungsten, respectively. Synthesis of transuranics gradually undermined this point of view. HP Pavilion DV4-1201AU Battery

By 1944 an observation that curium failed to exhibit oxidation states above 4 (whereas its supposed 6th period neighbor, platinum, can reach oxidation state of 7) prompted Glenn Seaborg to formulate a so-called "actinide hypothesis". Studies of known actinides and discoveries of further transuranic elements provided more data in support of this point of view,HP Pavilion DV4-1201TU Battery

but the phrase "actinide hypothesis" (the implication being that "hypothesis" is something that's not been decisively proven) remained in active use by scientists through the late 1950s.

At present, there are two major methods of producing isotopes of transplutonium elements: irradiation of the lighter elements with either neutrons or accelerated charged particles. HP Pavilion DV4-1201TX Battery

The first method is most important for applications, as only neutron irradiation using nuclear reactors allows the production of sizeable amounts of synthetic actinides; however, it is limited to relatively light elements. The advantage of the second method is that elements heavier than plutonium, as well as neutron-deficient isotopes, can be obtained, which are not formed during neutron irradiation.HP Pavilion DV4-1202AU Battery

In 1962–1966, there were attempts in the United States to produce transplutonium isotopes using a series of six underground nuclear explosions. Small samples of rock were extracted from the blast area immediately after the test to study the explosion products, but no isotopes with mass number greater than 257 could be detected, HP Pavilion DV4-1202TU Battery

despite predictions that such isotopes would have relatively long half-lives of ?-decay. This inobservation was attributed to spontaneous fission owing to the large speed of the products and to other decay channels, such as neutron emission and nuclear fission.HP Pavilion DV4-1202TX Battery

From actinium to neptunium

Uranium and thorium were the first actinides discovered. Uranium was identified in 1789 by the German chemist Martin Heinrich Klaproth in pitchblende ore. He named it after the planet Uranus,[6] which had been discovered only eight years earlier. HP Pavilion DV4-1203AU Battery

Klaproth was able to precipitate a yellow compound (likely sodium diuranate) by dissolving pitchblende in nitric acid and neutralizing the solution with sodium hydroxide. He then reduced the obtained yellow powder with charcoal, and extracted a black substance that he mistook for metal.HP Pavilion DV4-1203TU Battery

Only 60 years later, the French scientist Eugène-Melchior Péligot identified it with uranium oxide. He also isolated the first sample of uranium metal by heating uranium tetrachloride with potassium. The atomic mass of uranium was then calculated as 120, but Dmitri Mendeleev in 1872 corrected it to 240 using his periodicity laws. HP Pavilion DV4-1203TX Battery

This value was confirmed experimentally in 1882 by K. Zimmerman.

Thorium oxide was discovered by Friedrich Wöhler in the mineral, which was found in Norway (1827). Jöns Jacob Berzeliuscharacterized this material in more detail by in 1828. By reduction of thorium tetrachloride with potassium, he isolated the metal and named it thorium after the Norse god of thunder and lightning Thor.HP Pavilion DV4-1204AU Battery

The same isolation method was later used by Péligot for uranium.

Actinium was discovered in 1899 by André-Louis Debierne, an assistant of Marie Curie, in the pitchblende waste left after removal of radium and polonium. He described the substance (in 1899) as similar to titanium nd (in 1900) as similar to thorium.HP Pavilion DV4-1204TU Battery

The discovery of actinium by Debierne was however questioned in 1971 and 2000, arguing that Debierne's publications in 1904 contradicted his earlier work of 1899–1900. The name for word actinium comes from the Greek aktis, aktinos (?????, ???????), meaning beam or ray.HP Pavilion DV4-1204TX Battery

This metal was discovered not by its own radiation but by the radiation of the daughter products. Owing to the close similarity of actinium and lanthanum and low abundance, pure actinium could only be produced in 1950. The term actinide was probably introduced by Victor Goldschmidt in 1937.HP Pavilion DV4-1205AU Battery

Protactinium was possibly isolated in 1900 by William Crookes. It was first identified in 1913, when Kasimir Fajans and Oswald Helmuth Göhring encountered the short-lived isotope 234mPa (half-life 1.17 minutes) during their studies of the 238U decay. They named the new element brevium (from Latin brevis meaning brief);HP Pavilion DV4-1205TU Battery

the name was changed to protoactinium (from Greek ?????? + ????? meaning "first beam element") in 1918 when two groups of scientists, led by Otto Hahn and Lise Meitner of Germany and Frederick Soddy and John Cranston of Great Britain, independently discovered 231Pa. The name was shortened to Protactinium in 1949. HP Pavilion DV4-1205TX Battery

This element was little characterized until 1960, when A. G. Maddock and co-workers in UK produced 130 grams of protactinium from 60 tonnes of waste left after extraction of uranium from its ore.

Neptunium (named for the planet Neptune, the next planet out from Uranus, after which uranium was named) was discovered by Edwin McMillan and Philip H. Abelson in 1940 in Berkeley, California.HP Pavilion DV4-1206TU Battery

They produced the 239Np isotope (half-life 2.4 days) by bombarding uranium with slow neutrons.It was the first transuranium element produced synthetically.

Plutonium and above

Transuranium elements do not occur in sizeable quantities in nature and are commonly synthesized via nuclear reactions conducted with nuclear reactors.HP Pavilion DV4-1206TX Battery

For example, under irradiation with reactor neutrons, uranium-238 partially converts to plutonium-239:

In this way, Enrico Fermi with collaborators, using the first nuclear reactor Chicago Pile-1, obtained significant amounts of plutonium-239, which were then used in nuclear weapons.HP Pavilion DV4-1207TU Battery

Actinides with the highest mass numbers are synthesized by bombarding uranium, plutonium, curium and californium with ions of nitrogen, oxygen, carbon, neon or boron in a particle accelerator. So, nobelium was produced by bombarding uranium-238 with neon-22 as.HP Pavilion DV4-1207TX Battery

First isotopes of transplutonium elements, americium-241 and curium-242, were synthesized in 1944 by Glenn T. Seaborg, Ralph A. James and Albert Ghiorso.[39] Curium-242 was obtained by bombarding plutonium-239 with 32-MeV ?-particles

.The americium-241 and curium-242 isotopes also were produced by irradiating plutonium in a nuclear reactor.HP Pavilion DV4-1208TU Battery

The latter element was named after Marie Curie and her husband Pierre who are noted for discovering radium and for their work in radioactivity.

Bombarding curium-242 with ?-particles resulted in an isotope of californium 245Cf (1950), and a similar procedure yielded in 1949 berkelium-243 from americium-241.HP Pavilion DV4-1208TX Battery

The new elements were named after Berkeley, California, by analogy with its lanthanidehomologue terbium, which was named after the village of Ytterby in Sweden.

In 1945, B. B. Cunningham obtained the first bulk chemical compound of a transplutonium element, namely americium hydroxide.HP Pavilion DV4-1209TU Battery

Over the next three to four years, milligram quantities of americium and microgram amounts of curium were accumulated that allowed production of isotopes of berkelium (Thomson, 1949) and californium (Thomson, 1950). Sizeable amounts of these elements were produced only in 1958 (Burris B. Cunningham and Stanley G. Thomson), and the first californium compound (0.3 µg of CfOCl) was obtained only in 1960 by B. B. Cunningham and J. C. Wallmann).HP Pavilion DV4-1209TX Battery

Einsteinium and fermium were identified in 1952–1953 in the fallout from the "Ivy Mike" nuclear test (1 November 1952), the first successful test of a hydrogen bomb. Instantaneous exposure of uranium-238 to a large neutron flux resulting from the explosion produced heavy isotopes of uranium, including uranium-253 and uranium-255,HP Pavilion DV4-1210EA Battery

and their ?-decay yielded einsteinium-253 and fermium-255. The discovery of the new elements and the new data on neutron capture were initially kept secret on the orders of the U.S. military until 1955 due to Cold War tensions. Nevertheless, the Berkeley team were able to prepare einsteinium and fermium by civilian means, HP Pavilion DV4-1210EF Battery

through the neutron bombardment of plutonium-239, and published this work in 1954 with the disclaimer that it was not the first studies that had been carried out on the elements. The "Ivy Mike" studies were declassified and published in 1955.[51] The first significant (submicrograms) amounts of einsteinium were produced in 1961 by Cunningham and colleagues, but this has not been done for fermium yet.HP Pavilion DV4-1210EO Battery

The first isotope of mendelevium, 256Md (half-life 87 min), was synthesized by Albert Ghiorso, Glenn T. Seaborg, Gregory R. Choppin, Bernard G. Harvey and Stanley G. Thompson when they bombarded an 253Es target with alpha particles in the 60-inch cyclotron of Berkeley Radiation LaboratoryHP Pavilion DV4-1210ER Battery

; this was the first isotope of any element to be synthesized one atom at a time.[56]

There were several attempts to obtain isotopes of nobelium by Swedish (1957) and American (1958) groups, but the first reliable results was the synthesis of 256No by the Russian group (Georgy Flyorov et al.) in 1965, as acknowledges by the IUPAC in 1992. In their experiments, Flyorov et al. bombarded uranium-238 with neon-22.[8]HP Pavilion DV4-1210TU Battery

In 1961, Ghiorso et al. obtained the first isotope of lawrencium by irradiating californium (mostly californium-252) with boron-10 and boron-11 ions.[8] The mass number of this isotope was not clearly established (possibly 258 or 259) at the time. In 1965, 256Lr were synthesized by Flyorov et al. from 243Am and 18O.HP Pavilion DV4-1210TX Battery

Thus IUPAC recognized the nuclear physics teams at Dubna and Berkeley as the co-discoverers of lawrencium.

Thirty one isotopes of actinium and eight excited isomeric states of some of its nuclides were identified by 2010.HP Pavilion DV4-1211EA Battery

Three isotopes, 225Ac, 227Ac and 228Ac, were found in nature and the others were produced in the laboratory; only the three natural isotopes are used in applications. Actinium-225 is a member of radioactiveneptunium series;[61] it was first discovered in 1947 as a fission product of uranium-233, it is an ?-emitter with a half-life of 10 days. HP Pavilion DV4-1211TU Battery

Actinium-225 is less available than actinium-228, but is more promising in radiotracer applications.[29] Actinium-227 (half-life 21.77 years) occurs in all uranium ores, but in small quantities. One gram of uranium (in radioactive equilibrium) contains only 2×10?10 gram of 227Ac.HP Pavilion DV4-1211TX Battery

Actinium-228 is a member of radioactive thorium series formed by the decay of 228Ra;[61] it is a ?– emitter with a half-life of 6.15 hours. In one tonne of thorium there is 5×10?8 gram of 228Ac. It was discovered by Otto Hahn in 1906.[29]

Twenty nine isotopes of protactinium are known with mass numbers 212–240 as well as three excited isomeric states. HP Pavilion DV4-1212LA Battery

Only 231Pa and 234Pa have been found in nature. All the isotopes have short lifetime, except for protactinium-231 (half-life 32,760 years). The most important isotopes are 231Pa and 233Pa, which is an intermediate product in obtaining uranium-233 and is the most affordable among artificial isotopes of protactinium. HP Pavilion DV4-1212TU Battery

233Pa has convenient half-life and energy of ?-radiation, and thus was used in most studies of protactinium chemistry. Protactinium-233 is a ?-emitter with a half-life of 26.97 days.[57][62]

Uranium has the highest number (25) of both natural and synthetic isotopes. They have mass numbers of 217–242,[58] and three of them, 234U, 235U and 238U, are present in appreciable quantities in nature.HP Pavilion DV4-1212TX Battery

Among others, the most important is 233U, which is a final product of transformations of 232Th irradiated by slow neutrons. 233U has a very higher fission efficiency by low-energy (thermal) neutrons, compared e.g. with 235U. Most uranium chemistry studies were carried out on uranium-238 owing to its long half-life of 4.4×109 years.[HP Pavilion DV4-1213LA Battery

There are 19 isotopes of neptunium with mass numbers from 225 to 244;[58] they are all highly radioactive. The most popular among scientists are long-lived 237Np (t½ = 2.20×106 years) and short-lived 239Np, 238Np (t½ ~ 2 days).[37]

Sixteen isotopes of americium are known with mass numbers from 232 to 248.HP Pavilion DV4-1213TU Battery

The most important are 241Am and 243Am, which are alpha-emitters and also emit soft, but intense ?-rays; both of them can be obtained in an isotopically pure form. Chemical properties of americium were first studied with 241Am, but later shifted to243Am, which is almost 20 times less radioactive. HP Pavilion DV4-1213TX Battery

The disadvantage of 243Am is production of the short-lived daughter isotope 239Np, which has to be considered in the data analysis.[64]

Among 19 isotopes of curium,[58] the most accessible are 242Cm and 244Cm; they are ?-emitters, but with much shorter lifetime than the americium isotopes. HP Pavilion DV4-1214LA Battery

These isotopes emit almost no ?-radiation, but undergo spontaneous fission with the associated emission of neutrons. More long-lived isotopes of curium (245–248Cm, all ?-emitters) are formed as a mixture during neutron irradiation of plutonium or americium. Upon short irradiation, this mixture is dominated by curium-246, and then curium-248 begins to accumulate.HP Pavilion DV4-1214TU Battery

Both of these isotopes, especially 248Cm, have a longer half-life (3.48×105 years) and are much more convenient for carrying out chemical research than 242Cm and 244Cm, but they also have a rather high rate of spontaneous fission. 247Cm has the longest lifetime among isotopes of curium (1.56×107 years), but is not formed in large quantities because of the strong fission induced by thermal neutrons.HP Pavilion DV4-1214TX Battery

Fourteen isotopes of berkelium were identified with mass numbers 238–252.[58] Only 249Bk is available in large quantities; it has a relatively short half-life of 330 days and emits mostly soft ?-particles, which are inconvenient for detection. Its alpha radiation is rather weak (1.45×10?3% with respect to ?-radiation), but is sometimes used to detect this isotope. HP Pavilion DV4-1215EF Battery

247Bk is an alpha-emitter with a long half-life of 1,380 years, but it is hard to obtain in appreciable quantities; it is not formed upon neutron irradiation of plutonium because of the ?-stability of isotopes of curium isotopes with mass number below 248.[64]

Isotopes of californium with mass numbers 237–256 are formed in nuclear reactors;[58] californium-253 is a ?-emitter and the rest are ?-emitters. HP Pavilion DV4-1215TU Battery

The isotopes with even mass numbers (250Cf, 252Cf and 254Cf) have a high rate of spontaneous fission, especially 254Cf of which 99.7% decays by spontaneous fission. Californium-249 has a relatively long half-life (352 years), weak spontaneous fission and strong ?-emission that facilitates its identification. HP Pavilion DV4-1215TX Battery

249Cf is not formed in large quantities in a nuclear reactor because of the slow ?-decay of the parent isotope 249Bk and a large cross section of interaction with neutrons, but it can be accumulated in the isotopically pure form as the ?-decay product of (pre-selected) 249Bk. HP Pavilion DV4-1216TU Battery

Californium produced by reactor-irradiation of plutonium mostly consists of 250Cf and252Cf, the latter being predominant for large neutron fluences, and its study is hindered by the strong neutron radiation.

Among the 16 known isotopes of einsteinium with mass numbers from 241 to 257[58] the most affordable is 253Es.HP Pavilion DV4-1216TX Battery

It is an ?-emitter with a half-life of 20.47 days, a relatively weak ?-emission and small spontaneous fission rate as compared with the isotopes of californium. Prolonged neutron irradiation also produces a long-lived isotope 254Es (t½ = 275.5 days).[65]

Nineteen isotopes of fermium are known with mass numbers of 242–260. 254Fm, 255Fm and 256Fm are ?-emitters with a short half-life (hours), which can be isolated in significant amounts. HP Pavilion DV4-1217LA Battery

257Fm (t½ = 100 days) can accumulate upon prolonged and strong irradiation. All these isotopes are characterized by high rates of spontaneous fission.[65][67]

Among the 15 known isotopes of mendelevium (mass numbers from 245 to 260),[58] the most studied is 256Md, which mainly decays through the electron capture (?-radiation is ?10%) with the half-life of 77 minutes. HP Pavilion DV4-1217TX Battery

Another alpha emitter, 258Md, has a half-life of 53 days. Both these isotopes are produced from rare einsteinium (253Es and 255Es respectively), that limits their so their availability.[57]

Long-lived isotopes of nobelium and isotopes of lawrencium (and of heavier elements) have relatively small half-lives.HP Pavilion DV4-1218TX Battery

For nobelium 11 isotopes are known with mass numbers 250–260 and 262. Chemical properties of nobelium and lawrencium were studied with 255No (t½ = 3 min) and 256Lr (t½ = 35 s). The longest-lived nobelium isotope 259No has a half-life of 1.5 hours.HP Pavilion DV4-1219TX Battery

Distribution in nature

Thorium and uranium are the most abundant actinides in nature with the respective mass concentrations of 1.6×10?3% and 4×10?4%. Uranium mostly occurs in the Earth's crust as a mixture of its oxides in the minerals uraninite, which is also called pitchblende because of its black color. HP Pavilion DV4-1220BR Battery

There are several dozens of other uranium minerals such as carnotite (KUO2VO4·3H2O) and autunite (Ca(UO2)2(PO4)2·nH2O). The isotopic composition of natural uranium is 238U (relative abundance 99.2742%), 235U (0.7204%) and 234U (0.0054%); of these 238U has the largest half-life of 4.51×109 years.HP Pavilion DV4-1220TX Battery

The worldwide production of uranium in 2009 amounted to 50,572 tonnes, of which 27.3% was mined in Kazakhstan. Other important uranium mining countries are Canada (20.1%), Australia (15.7%), Namibia (9.1%), Russia (7.0%), andNiger (6.4%).

The most abundant thorium minerals are thorianite (ThO2), thorite(ThSiO4) and monazite, ((Th,Ca,Ce)PO4).HP Pavilion DV4-1220US Battery

Most thorium minerals contain uranium and vice versa; and they all have significant fraction of lanthanides. Rich deposits of thorium minerals are located in the United States (440,000 tonnes), Australia and India (~300,000 tonnes each) and Canada (~100,000 tonnes).[HP Pavilion DV4-1221TX Battery

The abundance of actinium in the Earth's crust is only about 5×10?15%.[62] Actinium is mostly present in uranium-containing, but also in other minerals, though in much smaller quantities. The content of actinium in most natural objects corresponds to the isotopic equilibrium of parent isotope 235U, and it is not affected by the weak Ac migration.[HP Pavilion DV4-1222LA Battery]

Protactinium is more abundant (10?12%) in the Earth's crust than actinium. It was discovered in the uranium ore in 1913 by Fajans and Göhring.[33] As actinium, the distribution of protactinium follows that of 235U.

The half-life of the longest-lived isotope of neptunium, 237Np, is negligible compared to the age of the Earth.HP Pavilion DV4-1222NR Battery

Thus neptunium is present in nature in negligible amounts produced as intermediate decay products of other isotopes.[37] Traces of plutonium in uranium minerals were first found in 1942, and the more systematic results on 239Pu are summarized in the table (no other plutonium isotopes could be detected in those samples). HP Pavilion DV4-1222TX Battery

The upper limit of abundance of the longest-living isotope of plutonium, 244Pu, is 3×10?20%. Plutonium could not be detected in samples of lunar soil. Owing to its scarcity in nature, most plutonium is produced synthetically.

Extraction

Owing to the low abundance of actinides, their extraction is a complex, multistep process.HP Pavilion DV4-1223LA Battery

Fluorides of actinides are usually used because they are insoluble in water and can be easily separated with redox reactions.

Among the actinides, thorium and uranium are the easiest to isolate. Thorium is extracted mostly from monazite: thorium diphosphate (Th(PO4)2) is reacted with nitric acid, and the produced thorium nitrate treated with tributyl phosphate.HP Pavilion DV4-1223TX Battery

Rare-earthimpurities are separated by increasing the pH in sulfate solution.

In another extraction method, monazite is decomposed with a 45% aqueous solution of sodium hydroxide at 140 °C. Mixed metal hydroxides are extracted first, filtered at 80 °C, washed with water and dissolved with concentrated hydrochloric acid.HP Pavilion DV4-1224LA Battery

Next, the acidic solution is neutralized with hydroxides to pH = 5.8 that results in precipitation of thorium hydroxide (Th(OH)4) contaminated with ~3% of rare-earth hydroxides; the rest of rare-earth hydroxides remains in solution. Thorium hydroxide is dissolved in an inorganic acid and then purified from the rare earth elements.HP Pavilion DV4-1224TX Battery

An efficient method is the dissolution of thorium hydroxide in nitric acid, because the resulting solution can be purified by extraction with organic solvents:

Th(OH)4 + 4 HNO3 ? Th(NO3)4 + 4 H2O

Metallic thorium is separated from the anhydrous oxide, chloride or fluoride by reacting it with calcium in an inert atmosphere:HP Pavilion DV4-1225DX Battery

ThO2 + 2 Ca ? 2 CaO + Th

Sometimes thorium is extracted by electrolysis of a fluoride in a mixture of sodium and potassium chloride at 700–800 °C in a graphite crucible. Highly pure thorium can be extracted from its iodide with the crystal bar process.

Uranium is extracted from its ores in various ways. In one method, the ore is burned and then reacted with nitric acid to convert uranium into a dissolved state. HP Pavilion DV4-1225EE Battery

Treating the solution with a solution of tributyl phosphate (TBP) in kerosene transforms uranium into an organic form UO2(NO3)2(TBP)2. The insoluble impurities are filtered and the uranium is extracted by reaction with hydroxides as (NH4)2U2O7 or with hydrogen peroxide as UO4·2H2O.HP Pavilion DV4-1225LA Battery

When the uranium ore is rich in such minerals as dolomite, magnesite, etc., those minerals consume much acid. In this case, the carbonate method is used for uranium extraction. Its main component is an aqueous solution of sodium carbonate, which converts uranium into a complex [UO2(CO3)3]4–, which is stable in aqueous solutions at low concentrations of hydroxide ions.HP Pavilion DV4-1225TX Battery

The advantages of the sodium carbonate method are that the chemicals have low corrosivity (compared to nitrates) and that most non-uranium metals precipitate from the solution. The disadvantage is that tetravalent uranium compounds precipitate as well. Therefore, the uranium ore is treated with sodium carbonate at elevated temperature and under oxygen pressure:HP Pavilion DV4-1226TX Battery

2 UO2 + O2 + 6 CO32– ? 2 [UO2(CO3)3]4–

This equation suggests that the best solvent for the uranium carbonate processing is a mixture of carbonate with bicarbonate. At high pH, this results in precipitation of diuranate, which is treated with hydrogen in the presence of nickel yielding an insoluble uranium tetracarbonate.HP Pavilion DV4-1227LA Battery

Another separation method uses polymeric resins as a polyelectrolyte. Ion exchange processes in the resins result in separation of uranium. Uranium from resins is washed with a solution ofammonium nitrate or nitric acid that yields uranyl nitrate, UO2(NO3)2·6H2O. When heated, it turns into UO3, which is converted to UO2 with hydrogen:HP Pavilion DV4-1227TX Battery

UO3 + H2 ? UO2 + H2O

Reacting uranium dioxide with fluoric acid changes it to uranium tetrafluoride, which yields uranium metal upon reaction with magnesium metal:[76]

4 HF + UO2 ? UF4 + 2 H2O

To extract plutonium, neutron-irradiated uranium is dissolved in nitric acid, and a reducing agent (FeSO4, or H2O2) is added to the resulting solution. HP Pavilion DV4-1227US Battery

This addition changes the oxidation state of plutonium from +6 to +4, while uranium remains in the form of uranyl nitrate (UO2(NO3)2). The solution is treated with a reducing agent and neutralized with ammonium carbonate to pH = 8 that results in precipitation of Pu4+ compounds.HP Pavilion DV4-1228CA Battery

In another method, Pu4+and UO22+ are first extracted with tributyl phosphate, then reacted with hydrazine washing out the recovered plutonium.[74]

The major difficulty in separation of actinium is the similarity of its properties with those of lanthanum. Thus actinium is either synthesized in nuclear reactions from isotopes of radium or separated using ion-exchange procedures.HP Pavilion DV4-1228LA Battery

Properties

Actinides have similar properties to lanthanides. The 6d and 7s electronic shells are completed in actinium and thorium, and the 5fshell is being filled with further increase in atomic number; the 4f shell is filled in the lanthanides. HP Pavilion DV4-1228TX Battery

The first experimental evidence for the filling of the 5f shell in actinides was obtained by McMillan and Abelson in 1940. As in lanthanides (see lanthanide contraction), the ionic radius of actinides monotonically decreases with atomic number (see also Aufbau principle).HP Pavilion DV4-1229LA Battery

Actinides are typical metals. All of them are soft and have a silvery color (but tarnish in air), relatively high density and plasticity. Some of them can be cut with a knife. Their electrical resistivity varies between 15 and 150 µOhm·cm. The hardness of thorium is similar to that of softsteel, so heated pure thorium can be rolled in sheets and pulled into wire. HP Pavilion DV4-1229TX Battery

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