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4 décembre 2011 7 04 /12 /décembre /2011 03:51

Thorium is nearly half as dense as uranium and plutonium, but is harder than either of them. All actinides are radioactive,paramagnetic, and, with the exception of actinium, have several crystalline phases: plutonium has seven, and uranium, neptunium and californium three. HP Pavilion DV4-1230BR Battery

Crystal structures of protactinium, uranium, neptunium and plutonium do not have clear analogs among the lanthanide and are more similar to those of the 3d-transition metals.

All actinides are pyrophoric, especially when finely divided, that is, they spontaneously ignite upon reaction with air.HP Pavilion DV4-1230TX Battery

The melting point of actinides does not have a clear dependence on the number of f-electrons. The unusually low melting point of neptunium and plutonium (~640 °C) is explained by hybridization of 5f and 6dorbitals and the formation of directional bonds in these metals.HP Pavilion DV4-1231TX Battery

Chemical properties

Like the lanthanides, all actinides are highly reactive with halogens and chalcogens; however, actinides react more easily. Actinides, especially those with a small number of 5f-electrons, are prone to hybridization. This is explained by the similarity of the electron energies at the 5f, 7s and 6d shells. HP Pavilion DV4-1232TX Battery

Most actinides exhibit a larger variety of valence states, and the most stable are +6 for uranium, +5 for protactinium and neptunium, +4 for thorium and plutonium and +3 for actinium and other actinides.

Chemically, actinium is similar to lanthanum, which is explained by their similar ionic radii and electronic structure. HP Pavilion DV4-1233TX Battery

Like lanthanum, actinium has oxidation of +3, but it is less reactive and has more pronounced basic properties. Among other trivalent actinides Ac3+ is least acidic, i.e. has the weakest tendency to hydrolyze in aqueous solutions.

Thorium is rather active chemically. Owing to lack of electrons on 6d-and 4f-orbitals, the tetravalent thorium compounds are colorless. HP Pavilion DV4-1234TX Battery

At pH < 3, the solutions of thorium salts are dominated by the cations [Th(H2O)8]4+. The Th4+ ion is relatively large, and depending on the coordination number can have a radius between 0.95 and 1.14 Å. As a result, thorium salts have a weak tendency to a hydrolysis. HP Pavilion DV4-1235EE Battery

The distinctive ability of thorium salts is their high solubility, not only in water, but also in polar organic solvents.

Protactinium exhibits two valence states; the +5 is stable, and the +4 state easily oxidizes to protactinium(V). Thus tetravalent protactinium in solutions is obtained by the action of strong reducing agents in a hydrogen atmosphere. HP Pavilion DV4-1235EI Battery

Tetravalent protactinium is chemically similar to uranium(IV) and thorium(IV). Fluorides, phosphates, hypophosphate, iodate and phenylarsonates of protactinium(IV) are insoluble in water and dilute acids. Protactinium forms soluble carbonates. The hydrolytic properties of pentavalent protactinium are close to those of tantalum(V) and niobium(V). HP Pavilion DV4-1235TX Battery

The complex chemical behavior of protactinium is a consequence of the start of the filling of the 5f shell in this element.

Uranium has a valence from 3 to 6, the last being most stable. In the hexavalent state, uranium is very similar to the sixth group elements. Many compounds of uranium(IV) and uranium(VI) are nonstoichiometric, i.e. have variable composition. HP Pavilion DV4-1236TX Battery

For example, the actual chemical formula of uranium dioxide is UO2+x, where xvaries between ?0.4 and 0.32. Uranium(VI) compounds are weak oxidants. Most of them contain the linear "uranyl" group, UO22+. Between 4 to 6 ligands can be accommodated in an equatorial plane perpendicular to the uranyl group. HP Pavilion DV4-1237TX Battery

The uranyl group acts as a hard acid and forms stronger complexes with oxygen-donor ligands than with nitrogen-donor ligands. NpO22+ and PuO22+ are also the common form of Np and Pu in the +6 oxidation state. Uranium(IV) compounds exhibit reducing properties, e.g., they are easily oxidized by atmospheric oxygen. HP Pavilion DV4-1238TX Battery

Uranium(III) is a very strong reducing agent. Owing to the presence of d-shell, uranium (as well as many other actinides) forms organometallic compounds, such as UIII(C5H5)3 and UIV(C5H5)4.

Neptunium has valence states from 3 to 7, which can be simultaneously observed in solutions. HP Pavilion DV4-1239TX Battery

The most stable state in solution is +5, but the valence +4 is preferred in solid neptunium compounds. Neptunium metal is very reactive. Ions of neptunium are prone to hydrolysis and formation of coordination compounds.

Plutonium also exhibits the valence between 3 and 7, and thus is chemically similar to neptunium and uranium.HP Pavilion DV4-1240TX Battery

It is highly reactive, and quickly forms an oxide film in air. Plutonium reacts with hydrogen even at temperatures as low as 25–50 °C; it also easily forms halides and intermetallic compounds. Hydrolysis reactions of plutonium ions of different oxidation states are quite diverse. Plutonium(V) can enter polymerization reactions.HP Pavilion DV4-1241TX Battery

The largest chemical diversity among actinides is observed in americium, which can have valence between 2 and 6. Divalent americium is obtained only in dry compounds and non-aqueous solutions (acetonitrile). Oxidation states +3, +5 and +6 are typical for aqueous solutions, but also in the solid state. HP Pavilion DV4-1242TX Battery

Tetravalent americium forms stable solid compounds (dioxide, fluoride and hydroxide) as well as complexes in aqueous solutions. It was reported that in alkaline solution americium can be oxidized to the heptavalent state, but these data proved erroneous. The most stable valence of americium is 3 in the aqueous solutions and 3 or 4 in solid compounds.HP Pavilion DV4-1243TX Battery

Valence 3 is dominant in all subsequent elements up to lawrencium (with the possible exception of nobelium). Curium can be tetravalent in solids (fluoride, dioxide). Berkelium, along with a valence of +3, also shows the valence of +4, more stable than that of curium; the valence 4 is observed in solid fluoride and dioxide.HP Pavilion DV4-1244TX Battery

The stability of Bk4+ in aqueous solution is close to that of Ce4+. Only valence 3 was observed for californium, einsteinium and fermium. The divalent state is proven for mendelevium and nobelium, and in nobelium it is more stable than the trivalent state. Lawrencium shows valence 3 both in solutions and solids.HP Pavilion DV4-1245BR Battery

The redox potential   increases from ?0.32 V in uranium, through 0.34 V (Np) and 1.04 V (Pu) to 1.34 V in americium revealing the increasing reduction ability of the An4+ ion from americium to uranium. All actinides form AnH3 hydrides of black color with salt-like properties.HP Pavilion DV4-1245TX Battery

Actinides also produce carbides with the general formula of AnC or AnC2 (U2C3 for uranium) as well as sulfides An2S3 and AnS2.

Some actinides can exists in various oxide forms: An2O3, AnO2, An2O5 and AnO3. For all actinides, oxides AnO3are amphoteric and An2O3, AnO2 and An2O5 are basic, they easily react with water, forming bases:HP Pavilion DV4-1246TX Battery

An2O3 + 3 H2O ? 2 An(OH)3.

These bases are poorly soluble in water and by their activity are close to the hydroxides of rare-earth metals. The strongest base is of actinium. All compounds of actinium are colorless, except for black actinium sulfide (Ac2S3).[84] Dioxides of tetravalent actinides crystallize in the cubic system, same as in calcium fluoride.HP Pavilion DV4-1247TX Battery

Thorium reacting with oxygen exclusively forms dioxide:

Thorium dioxide is a refractory material with the highest melting point among any known oxide (3390 °C). Adding 0.8–1% ThO2 to tungsten stabilizes its structure, so the doped filaments have better mechanical stability to vibrations. HP Pavilion DV4-1248TX Battery

To dissolve ThO2 in acids, it is heated to 500–600 °C; heating above 600 °C produces a very resistant to acids and other reagents form of ThO2. Small addition of fluoride ions catalyses dissolution of thorium dioxide in acids.

Two protactinium oxides were obtained: PaO2 (black) and Pa2O5(white); the former is isomorphic with ThO2 and the latter is easier to obtain. Both oxides are basic, and Pa(OH)5 is a weak, poorly soluble base.HP Pavilion DV4-1249TX Battery

Decomposition of certain salts of uranium, for example UO2(NO3)·6H2O in air at 400 °C, yields orange or yellow UO3.[93] This oxide is amphoteric and forms several hydroxides, the most stable being UO2(OH)2. Reaction of uranium(VI) oxide with hydrogen results in uranium dioxide, which is similar in its properties with ThO2. HP Pavilion DV4-1250TX Battery

This oxide is also basic and corresponds to the uranium hydroxide (U(OH)4).

Plutonium, neptunium and americium form two basic oxides: An2O3 and AnO2. Neptunium trioxide is unstable, thus only Np3O8could be obtained so far. However, the oxides of plutonium and neptunium with the chemical formula AnO2 and An2O3 are well characterized.HP Pavilion DV4-1251TX Battery

Actinides easily react with halogens forming salts with the formulas MX3 and MX4 (X = halogen). So the first berkelium compound, BkCl3, was synthesized in 1962 with an amount of 3 nanogram. Like the halogens of rare earth elements, actinide chlorides, bromides, and iodides are water soluble, and fluorides are insoluble.HP Pavilion DV4-1252TX Battery

Uranium easily yields a colorless hexafluoride, which sublimates at a temperature of 56.5 °C; because of its volatility, it is used in the separation of uranium isotopes with gas centrifuge or gaseous diffusion. Actinide hexafluorides have properties close to anhydrides. They are very sensitive to moisture and hydrolyze forming AnO2F2.HP Pavilion DV4-1253TX Battery

Pentachloride and black hexachloride of uranium were synthesized, but they are both unstable.

Action of acids on actinides yields salts, and if the acids are non-oxidizing then the actinide in the salt is in low-valence state:HP Pavilion DV4-1254TX Battery

U + 2 H2SO4 ? U (SO4)2 + 2 H2

2 Pu + 6 HCl ? 2 PuCl3 + 3 H2

However, in these reactions the regenerating hydrogen can react with the metal, forming the corresponding hydride. Uranium reacts with acids and water much easier than thorium.HP Pavilion DV4-1255TX Battery

Actinide salts can also be obtained by dissolving the corresponding hydroxides in acids. Nitrates, chlorides, sulfates and perchlorates of actinides are water soluble. When crystallizing from aqueous solutions, these salts forming a hydrates, such as Th(NO3)4·6H2O, Th(SO4)2·9H2O and Pu2(SO4)3·7H2O. Salts of high-valence actinides easily hydrolyze.HP Pavilion DV4-1256TX Battery
So, colorless sulfate, chloride, perchlorate and nitrate of thorium transform into basic salts with formulas Th(OH)2SO4 and Th(OH)3NO3. The solubility and insolubility of trivalent and tetravalent actinides is like that of lanthanide salts.HP Pavilion DV4-1257TX Battery

So phosphates, fluorides, oxalates, iodates and carbonatesof actinides are weakly soluble in water; they precipitate as hydrates, such as ThF4·3H2O and Th(CrO4)2·3H2O.

Actinides with oxidation state +6, except for the AnO22+-type cations, form [AnO4]2–, [An2O7]2– and other complex anions. HP Pavilion DV4-1258TX Battery

For example, uranium, neptunium and plutonium form salts of the Na2UO4 (uranate) and (NH4)2U2O7 (diuranate) types. In comparison with lanthanides, actinides easier form coordination compounds, and this ability increases with the actinide valence.HP Pavilion DV4-1259TX Battery

Trivalent actinides do not form fluoride coordination compounds, whereas tetravalent thorium forms K2ThF6, KThF5, and even K5ThF9 complexes. Thorium also forms the corresponding sulfates (for example Na2SO4·Th (SO4)2·5H2O), nitrates and thiocyanates. HP Pavilion DV4-1260TX Battery

Salts with the general formula An2Th(NO3)6·nH2O are of coordination nature, with the coordination number of thorium equal to 12. Even easier is to produce complex salts of pentavalent and hexavalent actinides. The most stable coordination compounds of actinides – tetravalent thorium and uranium – are obtained in reactions with diketones, e.g.acetylacetone.HP Pavilion DV4-1265DX Battery

Applications

While actinides have some established daily-life applications, such as in smoke detectors (americium) ]and gas mantles(thorium),[76] they are mostly used in nuclear weapons and use as a fuel in nuclear reactors.HP Pavilion DV4-1272CM Battery

The last two areas exploit the property of actinides to release enormous energy in nuclear reactions, which under certain conditions may become self-sustainingchain reaction.

The most important isotope for nuclear power applications is uranium-235. It is used in thethermal reactor, and its concentration in natural uranium does not exceed 0.72%. HP Pavilion DV4-1275MX Battery

This isotope strongly absorbs thermal neutrons releasing much energy. One fission act of 1 gram of 235U converts into about 1 MW·day. Of importance, is that 235U emits more neutrons than it absorbs;[101] upon reaching the critical mass, 235U enters into a self-sustaining chain reaction.HP Pavilion DV4-1280US Battery

Typically, uranium nucleus is divided into two fragments with the release of 2–3 neutrons, for example:

Other promising actinide isotopes for nuclear power are thorium-232 and its fission product, formed uranium-233.HP Pavilion DV4-1281US Battery

Emission of neutrons during the fission of uranium is important not only for maintaining the nuclear chain reaction, but also for the synthesis of the heavier actinides. Uranium-239 converts via ?-decay into plutonium-239, which, like uranium-235, is capable of spontaneous fission. HP Pavilion DV4-1282CM Battery

The world's first nuclear reactors were built not for energy, but for producing plutonium-239 for nuclear weapons.

About half of the produced thorium is used as the light-emitting material of gas mantles. Thorium is also added into multicomponent alloys of magnesium and zinc. HP Pavilion DV4-1283CL Battery

So the Mg-Th alloys are light and strong, but also have high melting point and ductility and thus are widely used in the aviation industry and in the production of missiles. Thorium also has good electron emissionproperties, with long lifetime and low potential barrier for the emission.HP Pavilion DV4-1287CL Battery

The relative content of thorium and uranium isotopes is widely used to estimate the age of various objects, including stars (see radiometric dating).

The major application of plutonium has been in nuclear weapons, where the isotope plutonium-239 was a key component due to its ease of fission and availability. HP Pavilion DV4-1290BR Battery

Plutonium-based designs allow reducing the critical mass to about a third of that for uranium-235. The "Fat Man"-type plutonium bombs produced during the Manhattan Project used explosive compression of plutonium to obtain significantly higher densities than normal, combined with a central neutron source to begin the reaction and increase efficiency. HP Pavilion DV4-1290EL Battery

Thus only 6.2 kg of plutonium was needed for anexplosive yield equivalent to 20 kilotons of TNT. (See also Nuclear weapon design.) Hypothetically, as little as 4 kg of plutonium—and maybe even less—could be used to make a single atomic bomb using very sophisticated assembly designs.HP Pavilion DV4-1292CM Battery

Plutonium-238 is potentially more efficient isotope for nuclear reactors, as it has smaller critical mass than uranium-235, but releases much more thermal energy (0.56 W/g). However, its application is limited by the high price (about 1000 USD/g). This isotope has been used in thermopiles and water distillationsystems of some space satellites and stations. HP Pavilion DV4-1299EE Battery

So Galileo and Apollo spacecrafts (e.g. Apollo 14 ) had heaters powered by kilogram quantities of plutonium-238 oxide; this heat is also transformed into electricity with thermopiles. The decay of plutonium-238 is produced relatively harmless alpha particles and is not accompanied by gamma-irradiation. HP Pavilion DV4-1300 CTO Battery

Therefore and this isotope (~160 mg) is used as the energy source in heart pacemakers where it lasts about 5 times longer than conventional batteries.

Actinium-227 is used as a neutron source. Its high specific energy (14.5 W/g) and the possibility of obtaining significant quantities of thermally stable compounds are attractive for use in long-lasting thermoelectric generators for remote use. HP Pavilion DV4-1301TU Battery

228Ac is used as an indicator of radioactivity in chemical research, as it emits high-energy electrons (2.18 MeV) that can be easily detected. 228Ac-228Ra mixtures are widely used as an intense gamma-source in industry and medicine.

Toxicity

Radioactive substances can harm human health via (i) local skin contamination, (ii) internal exposure due to ingestion of radioactive isotopes, and (iii) external overexposure by ?-activity and ?-radiation.HP Pavilion DV4-1301TX Battery

Together with radium and transuranium elements, actinium is one of the most dangerous radioactive poisons with high specific ?-activity. The most important feature of actinium is its ability to accumulate and remain in the surface layer of skeletons. At the initial stage of poisoning, actinium accumulates in the liver. HP Pavilion DV4-1302TU Battery

Another danger of actinium is that it undergoes radioactive decay faster than being excreted. Adsorptionfrom the digestive tract is much smaller (~0.05%) for actinium than radium.[

Protactinium in the body tends to accumulate in the kidneys and bones. The maximum safe dose of Pa in the human body is 0.03 µCithat corresponds to 0.5 micrograms of 231Pa. HP Pavilion DV4-1302TX Battery

This isotope, which might be present in the air as aerosol, is 2.5×108 times more toxic than hydrocyanic acid.[62]

Plutonium, when entering the body through air, food or blood (e.g. a wound), mostly settles in the lungs, liver and bones with only about 10% going to other organs, and remains there for decades.HP Pavilion DV4-1303TU Battery

The long residence time of plutonium in the body is partly explained by its poor solubility in water. Some isotopes of plutonium emit ionizing ?-radiation, which damages the surrounding cells. The median lethal dose (LD50) for 30 days in dogs after intravenous injection of plutonium is 0.32 milligram per kg of body mass, HP Pavilion DV4-1303TX Battery

and thus the lethal dose for humans is approximately 22 mg for a person weighing 70 kg; the amount for respiratory exposure should be approximately four times greater. Another estimate assumes that plutonium is 50 times less toxic than radium, and thus permissible content of plutonium in the body should be 5 µg or 0.3 µCi.HP Pavilion DV4-1304TU Battery

Such amount is nearly invisible in under microscope. After trials on animals, this maximum permissible dose was reduced to 0.65 µg or 0.04 µCi. Studies on animals also revealed that the most dangerous Pu exposure route is through inhalation, after which 5–25% of inhaled substances is retained in the body. HP Pavilion DV4-1304TX Battery

Depending on the particle size and solubility of the plutonium compounds, Pu is localized either in the lungs or in the lymphatic system, or is absorbed in the blood and then transported to the liver and bones. Contamination via food is the least likely way. In this case, only about 0.05% of soluble 0.01% insoluble compounds of plutonium absorbs into blood, and the rest is excreted. HP Pavilion DV4-1305TU Battery

Exposure of damaged skin to plutonium would retain nearly 100% of it.

The lanthanide or lanthanoid (IUPAC nomenclature) series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium.HP Pavilion DV4-1305TX Battery

These fifteen elements, along with chemically similar scandium andyttrium, are often collectively known as the rare earth elements.

The informal chemical symbol Ln is used in general discussions of lanthanide chemistry. All but one of the lanthanides are f-block elements, corresponding to the filling of the 4f electron shell; HP Pavilion DV4-1306TU Battery

lutetium, a d-block element, is also generally considered to be a lanthanide due to its chemical similarities with the other fourteen. All of the lanthanide elements form trivalent cations, Ln3+, whose chemistry is largely determined by the ionic radius, which decreases steadily from lanthanum to lutetium.HP Pavilion DV4-1306TX Battery

The lanthanide elements are the group of elements with atomic number increasing from 57 (lanthanum) to 71 (lutetium). They are termed lanthanide because the lighter elements in the series are chemically similar to lanthanum. Strictly speaking, both lanthanum and lutetium have been labeled as group 3 elements, because they both have a single valence electron in the d shell. HP Pavilion DV4-1307TU Battery

However, both elements are often included in any general discussion of the chemistry of the lanthanide elements.

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 seriesHP Pavilion DV4-1307TX Battery

(either lanthanum or lutetium, and either actinium or lawrencium, respectively) 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; HP Pavilion DV4-1308TU Battery

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).

Etymology

Together with scandium and yttrium, the trivial name "rare earths" is sometimes used to describe all the lanthanides. HP Pavilion DV4-1308TX Battery

This name arises from the minerals from which they were isolated, which were uncommon oxide-type minerals. However, the use of the name is deprecated by IUPAC, as the elements are neither rare in abundance nor "earths" (an obsolete term for water-insoluble strongly basic oxides of electropositive metals incapable of being smelted into metal using late 18th century technology).HP Pavilion DV4-1309TU Battery

Cerium is the 26th most abundant element in the Earth's crust, neodymium is more abundant than gold and even thulium (the least common naturally occurring lanthanide) is more abundant than iodine.HP Pavilion DV4-1309TX Battery

Despite their abundance, even the technical term "lanthanides" could be interpreted to reflect a sense of elusiveness on the part of these elements, as it comes from the Greek ????????? (lanthanein), "to lie hidden". However, if not referring to their natural abundance, but rather to their property of "hiding" behind each other in minerals, this interpretation is in fact appropriate. HP Pavilion DV4-1310TU Battery

The etymology of the term must be sought in the first discovery of lanthanum, at that time a so-called new rare earth element "lying hidden" in a cerium mineral, and it is an irony that lanthanum was later identified as the first in an entire series of chemically similar elements and could give name to the whole series. HP Pavilion DV4-1310TX Battery

The term lanthanide was probably introduced by Victor Goldschmidt in 1925.[7]

Chemical compounds

The electronic structure of the lanthanide elements, with minor exceptions is [Xe]6s24fn. In their compounds, the 6s electrons are lost and the ions have the configuration [Xe]4fm.HP Pavilion DV4-1311TU Battery

The chemistry of the lanthanides differs from main group elements and transition metals because of the nature of the 4f orbitals. These orbitals are "buried" inside the atom and are shielded from the atom's environment by the 4d and 5p electrons.HP Pavilion DV4-1311TX Battery

As a consequence of this, the chemistry of the elements is largely determined by their size, which decreases gradually from 102 pm (La3+) with increasing atomic number to 86 pm (Lu3+), the so-called lanthanide contraction. All the lanthanide elements exhibit the oxidation state +3.HP Pavilion DV4-1312TU Battery

In addition Ce3+ can lose its single f electron to form Ce4+with the stable electronic configuration of xenon. Also, Eu3+ can gain an electron to form Eu2+ with the f7configuration which has the extra stability of a half-filled shell. Promethium is effectively a man-made element as all its isotopes are radioactive with half-lives of less than 20 y.HP Pavilion DV4-1312TX Battery

In terms of reduction potentials, the Ln0/3+ couples are nearly the same for all lanthanides, ranging from -1.99 (for Eu) to -2.35 V (for Pr). Thus, these metals are highly reducing, with reducing power similar to alkaline earth metals such as Mg (-2.36 V).HP Pavilion DV4-1313DX Battery

Separation of lanthanides

The similarity in ionic radius between adjacent lanthanide elements makes it difficult to separate them from each other in naturally occurring ores and other mixtures. Historically, the very laborious processes of cascading and fractional crystallization was used.HP Pavilion DV4-1313TU Battery

Because the lanthanide ions have slightly different radii, the lattice energy of their salts and hydration energies of the ions will be slightly different, leading to a small difference in solubility. Salts of the formula Ln(NO3)3.2NH4NO3.4H2O can be used. Industrially, the elements are separated from each other by solvent extraction.HP Pavilion DV4-1313TX Battery

Typically an aqueous solution of nitrates is extracted into kerosene containing tri-n-butylphosphate, (BunO)3PO. The strength of the complexes formed increases as the ionic radius decreases, so solubility in the organic phase increases. Complete separation can be achieved continuously by use of countercurrent exchange methods.HP Pavilion DV4-1314TU Battery

The elements can also be separated by ion-exchange chromatography, making use of the fact that the stability constant for formation of EDTA complexes increases for log K ? 15.5 for [La(EDTA)]- to log K ? 19.8 for [Lu(EDTA)]-.

Ln(III) compounds

The trivalent lanthanides mostly form ionic salts. HP Pavilion DV4-1314TX Battery

The trivalent ions are hard acceptors and form more stable complexes with oxygen-donor ligands than with nitrogen-donor ligands. The larger ions are 9-coordinate in aqueous solution, [Ln(H2O)9]3+ but the smaller ions are 8-coordinate, [Ln(H2O)8]3+. There is some evidence that the later lanthanides have more water molecules in the second coordination sphere.HP Pavilion DV4-1315TU Battery

Complexation with monodentate ligands is generally weak because it is difficult to displace water molecules from the first coordination sphere. Stronger complexes are formed with chelating ligands because of the chelate effect, such as the tetra-anion derived from 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA).HP Pavilion DV4-1315TX Battery

Ln(II) and Ln(IV) compounds

The most common divalent derivatives of the lanthanides are for Eu(II), which achieves a favorable f7configuration. Divalent halide derivatives are known for all of the lanthanides. They are either conventional salts or are Ln(III) "electride"-like salts. HP Pavilion DV4-1316TU Battery

The simple salts include YbI2, EuI2, and SmI2. The electride-like salts, described as Ln2+, 2I-, e-, include NdI2, DyI2 and TmI2. Many of the iodides form soluble complexes with ethers, e.g. TmI2(dimethoxyethane)3.[11] Samarium(II) iodideis a useful reducing agent.HP Pavilion DV4-1316TX Battery

Ce(IV) in ceric ammonium nitrate is a useful oxidising agent. Otherwise tetravalent lanthanides are rare. The Ce(IV) is the exception owing to the tendency to form an unfilled f shell.

Organometallic compounds

Lanthanide-carbon ? bonds are well known but they exhibit carbanion-like behavior, unlike transition metal organometallic compounds.HP Pavilion DV4-1317TU Battery

Because of their large size, lanthanides tend to form more stable organometallic derivatives with bulky ligands to give compounds such as Ln[CH(SiMe3)3].[12] Similarly complexes ofcyclopentadienyl anion, e.g. [Ln(C5H5)3], are far less common than the corresponding pentamethylcyclopentadienyl, e.g. HP Pavilion DV4-1317TX Battery

Analogues of uranocene are derived from dilithiocyclooctatetraene, Li2C8H8. Organic lanthanide(II) compounds are also known, such as Cp*2Eu.

Magnetic and spectroscopic properties

All the trivalent lanthanide ions, except lutetium, have unpaired f electrons. However the magnetic moments deviate considerably from the spin-only values because of strong spin-orbit coupling. HP Pavilion DV4-1318TU Battery

The maximum number of unpaired electrons is 7, in Gd3+, with a magnetic moment of 7.94 B.M., but the largest magnetic moments, at 10.4-10.7 B.M., are exhibited by Dy3+ and Ho3+. However, in Gd3+ all the electrons have parallel spin and this property is important for the use of gadolinium complexes ascontrast reagent in MRI scans.HP Pavilion DV4-1318TX Battery

Crystal field splitting is rather small for the lanthanide ions and is less important than spin-orbit coupling in regard to energy levels.[13] Transitions of electrons between f orbitals are forbidden by the Laporte rule. Furthermore, because of the "buried" nature of the f orbitals, coupling with molecular vibrations is weak. HP Pavilion DV4-1319TU Battery

Consequently the spectra of lanthanide ions are rather weak and the absorption bands are similarly narrow. Glass containing holmium oxide and holmium oxide solutions (usually in perchloric acid) have sharp optical absorption peaks in the spectral range 200–900 nm and can be used as a wavelength calibration standard for optical spectrophotometers, and are available commercially.HP Pavilion DV4-1319TX Battery

As f-f transitions are Laporte-forbidden, once an electron has been excited, decay to the ground state will be slow. This makes them suitable for use in lasers as it makes the population inversion easy to achieve. The Nd:YAG laser is one that is widely used. Europium-doped yttrium vanadate was the first red phosphor to enable the development of colour television screens.HP Pavilion DV4-1320CA Battery

Lanthanide ions have notable luminescent properties due to their unique 4f orbitals. Laporte forbidden f-f transitions can be activated by excitation of a bound "antenna" ligand. This leads to sharp emission bands throughout the visible, NIR, and IR and relatively long luminescence lifetimes.HP Pavilion DV4-1320TX Battery

Geochemistry

The lanthanide contraction is responsible for the great geochemical divide that splits the lanthanides into light and heavy-lanthanide enriched minerals, the latter being almost inevitably associated with and dominated by yttrium. This divide is reflected in the first two "rare earths" that were discovered: yttria (1794) andceria (1803).HP Pavilion DV4-1321TX Battery

The geochemical divide has put more of the light lanthanides in the Earth's crust, but more of the heavy members in the Earth's mantle. The result is that although large rich ore-bodies are found that are enriched in the light lanthanides, correspondingly large ore-bodies for the heavy members are few. HP Pavilion DV4-1322TX Battery

The principal ores are monazite and bastnaesite. Monazite sands usually contain all the lanthanide elements, but the heavier elements are lacking in bastnaesite. The lanthanides obey the Oddo-Harkins rule - odd-numbered elements are less abundant than their even-numbered neighbours.HP Pavilion DV4-1322US Battery

Three of the lanthanide elements have radioactive isotopes with long half-lives (138La, 147Sm and 176Lu) that can be used to date minerals and rocks from Earth, the Moon and meteorites.

Biological effects

Compared to most other nondietary elements, non-radioactive lanthanides are classified as having low toxicity.HP Pavilion DV4-1323TX Battery

Technological applications

Lanthanide elements and their compounds have many uses but the quantities consumed are relatively small in comparison to other elements. About 15000 ton/year of the lanthanides are consumed as catalysts and in the production of glasses. HP Pavilion DV4-1324TX Battery

This 15000 tons corresponds to about 85% of the lanthanide production. From the perspective of value, however, applications in phosphors and magnets are more important.

Of the many technological devices, including superconductors, samarium-cobalt and neodymium-iron-boron high-flux rare-earth magnets, magnesium alloys, electronic polishers,HP Pavilion DV4-1325TX Battery

refining catalysts and hybrid car components (primarily batteries and magnets). Lanthanide ions are used as the active ions in luminescent materials used in optoelectronics applications, most notably the Nd:YAG laser. Erbium-doped fiber amplifiers are significant devices in optical-fiber communication systems.HP Pavilion DV4-1326TX Battery

Phosphors with lanthanide dopants are also widely used in cathode ray tube technology such as television sets. The earliest color television CRTs had a poor-quality red; europium as a phosphor dopant made good red phosphors possible. Yttrium iron garnet (YIG) spheres have been useful as tunable microwave resonators.HP Pavilion DV4-1327TX Battery

Lanthanide oxides are mixed with tungsten to improve their high temperature properties for welding, replacing thorium, which was mildly hazardous to work with. Many defense-related products also use lanthanide elements such as night vision goggles, rangefinders, the SPY-1 radar used in some Aegis equipped warships, and the propulsion system of Arleigh Burke-class destroyers all use rare earth elements in critical capacities. HP Pavilion DV4-1328TX Battery

Most lanthanides are widely used in lasers, and as (co-)dopants in doped-fiber optical amplifiers (e.g. Er-doped fiber amplfiers (EDFAs) which are used as repeaters in the terrestrial and submarine fiber-optic transmission links that carry internet traffic) . These elements deflect ultraviolet and infrared radiation and are commonly used in the production of sunglass lenses. HP Pavilion DV4-1329TX Battery

Other applications are summarized in the following table:

The complex [Gd(DOTA)]- is used in magnetic resonance imaging.

The lanthanide contraction is responsible for the great geochemical divide that splits the la

nthanides into light and heavy-lanthanide enriched minerals, the latter being almost inevitably associated with and dominated by yttrium.HP Pavilion DV4-1330TX Battery

This divide is reflected in the first two "rare earths" that were discovered: yttria (1794) andceria (1803). The geochemical divide has put more of the light lanthanides in the Earth's crust, but more of the heavy members in the Earth's mantle. The result is that although large rich ore-bodies are found that are enriched in the light lanthanides, HP Pavilion DV4-1331TX Battery

correspondingly large ore-bodies for the heavy members are few. The principal ores are monazite and bastnaesite. Monazite sands usually contain all the lanthanide elements, but the heavier elements are lacking in bastnaesite. The lanthanides obey the Oddo-Harkins rule - odd-numbered elements are less abundant than their even-numbered neighbours.HP Pavilion DV4-1332TX Battery

Three of the lanthanide elements have radioactive isotopes with long half-lives (138La, 147Sm and 176Lu) that can be used to date minerals and rocks from Earth, the Moon and meteorites.

Biological effects

Compared to most other nondietary elements, non-radioactive lanthanides are classified as having low toxicity.HP Pavilion DV4-1365DX Battery

Technological applications

Lanthanide elements and their compounds have many uses but the quantities consumed are relatively small in comparison to other elements. About 15000 ton/year of the lanthanides are consumed as catalysts and in the production of glasses.HP Pavilion DV4-1400 CTO Battery

This 15000 tons corresponds to about 85% of the lanthanide production. From the perspective of value, however, applications in phosphors and magnets are more important. HP Pavilion DV4-1404TX Battery

Of the many technological devices, including superconductors, samarium-cobalt and neodymium-iron-boron high-flux rare-earth magnets, magnesium alloys, electronic polishers, refining catalysts and hybrid car components (primarily batteries and magnets).HP Pavilion DV4-1405TX Battery

Lanthanide ions are used as the active ions in luminescent materials used in optoelectronics applications, most notably the Nd:YAG laser. Erbium-doped fiber amplifiers are significant devices in optical-fiber communication systems.Phosphors with lanthanide dopants are also widely used in cathode ray tube technology such as television sets. HP Pavilion DV4-1406TX Battery

The earliest color television CRTs had a poor-quality red; europium as a phosphor dopant made good red phosphors possible. Yttrium iron garnet (YIG) spheres have been useful as tunable microwave resonators. Lanthanide oxides are mixed with tungsten to improve their high temperature properties for welding, replacing thorium, which was mildly hazardous to work with.HP Pavilion DV4-1407TX Battery

Many defense-related products also use lanthanide elements such as night vision goggles, rangefinders, the SPY-1 radar used in some Aegis equipped warships, and the propulsion system of Arleigh Burke-class destroyers all use rare earth elements in critical capacities.HP Pavilion DV4-1408TX Battery

Most lanthanides are widely used in lasers, and as (co-)dopants in doped-fiber optical amplifiers (e.g. Er-doped fiber amplfiers (EDFAs) which are used as repeaters in the terrestrial and submarine fiber-optic transmission links that carry internet traffic) . These elements deflect ultraviolet and infrared radiation and are commonly used in the production of sunglass lensesHP Pavilion DV4-1409TX Battery

. Other applications are summarized in the following table:

The complex [Gd(DOTA)]- is used in magnetic resonance imaging.

Uranium in the environment refers to the science of the sources, environmental behaviour, and effects of uranium on humans and other animals. Uranium is weaklyradioactive and remains so because of its long physical half-life (4.468 billion years for uranium-238). HP Pavilion DV4-1413LA Battery

The biological half-life (the average time it takes for the human body to eliminate half the amount in the body) for uranium is about 15 days.[1] Normal functioning of the kidney, brain, liver, heart, and numerous other systems can be affected by uranium exposure, because uranium is a toxic metal.HP Pavilion DV4-1414LA Battery

The use of depleted uranium (DU) in munitions is controversial because of questions about potential long-term health effects.

Uranium is a naturally occurring element found in low levels within all rock, soil, and water. This is the highest-numbered element to be found naturally in significant quantities on earth.HP Pavilion DV4-1416LA Battery

According to the United Nations Scientific Committee on the Effects of Atomic Radiation the normal concentration of uranium in soil is 300 ?g/kg to 11.7 mg/kg.[5]

It is considered to be more plentiful than antimony, beryllium, cadmium, gold, mercury, silver, or tungsten and is about as abundant as tin, arsenic or molybdenum. HP Pavilion DV4-1425LA Battery

It is found in many minerals including uraninite (most common uranium ore), autunite,uranophane, torbernite, and coffinite. Significant concentrations of uranium occur in some substances such as phosphate rock deposits, and minerals such as lignite, and monazite sands in uranium-rich ores (it is recovered commercially from these sources).HP Pavilion DV4T Battery

Seawater contains about 3.3 parts per billion of uranium by weight (3.3 µg/kg)[6][not in citation given] as uranium(VI) forms solublecarbonate complexes. The extraction of uranium from seawater has been considered as a means of obtaining the element.HP Pavilion DV4T-1000 CTO Battery

Uranium is weakly radioactive and remains so because of its long physical half-life (4.468 billion years for uranium-238). Thebiological half-life (the average time it takes for the human body to eliminate half the amount in the body) for uranium is about 15 days.HP Pavilion DV4T-1100 CTO Battery

During the extraction of uranium ore and its processing, some releases of uranium occur. The releases of radium and other decay products of uranium are normally more important than the uranium in tailings ponds at the mines and ore processing centers.HP Pavilion DV4T-1200 CTO Battery

Depleted uranium (DU) is useful because of its very high density of 19.1 g/cm3 (68.4% denser than lead). Civilian uses include counterweights in aircraft, radiation shielding in medical radiation therapy and industrial radiography equipment, and containers used to transport radioactive materials.HP Pavilion DV4T-1200SE CTO Battery

Military uses include defensive armor plating and armor-piercing projectiles.

Uranium metal can disperse into the air and water, United Nations Environment Programme (UNEP) study says in part:

"The most important concern is the potential for future groundwater contamination by corroding penetrators (ammunition tips made out of DU).HP Pavilion DV4T-1300 CTO Battery

The munition tips recovered by the UNEP team had already decreased in mass by 10-15% in this way. This rapid corrosion speed underlines the importance of monitoring the water quality at the DU sites on an annual basis."HP Pavilion DV4T-1300SE CTO Battery

Combustion

Studies of depleted uranium aerosol exposure suggest that uranium combustion product particles would quickly settle out of the air,[9] and thus could not affect populations more than a few kilometres from target areas. HP Pavilion DV4T-1400 CTO Battery

The U.S. has admitted that there have been over 100 "friendly fire" incidents in which members of the U.S. military have been struck by DU munitions, and that an unknown number have been exposed to DU via inhalation of combustion products from burning DU munitions.HP Pavilion DV4Z Battery

Corrosion

It has been reported that the corrosion of uranium in a silica rich aqueous solution forms both uranium dioxide and uranium trioxide.

In pure water, schoepite {(UO2)8O2(OH)12.12(H2O)} is formed in the first week and then after four months studtite {(UO2)O2·4(H2O)} was formed.HP Pavilion DV4Z-1000 CTO Battery

Uranium metal reacts with water to form hydrogen gas, this reaction forms uranium dioxide and 2% to 9% uranium hydride. It is important to note that the rate of corrosion due to water is far greater than that caused by oxygen at temperatures around 100 °C (212 °F). HP Pavilion DV4Z-1100 CTO Battery

At pH values below 2 the corrosion rate at 100 °C goes down greatly, while as pH values go from 7 upwards the corrosion rate declines. Gamma irradiation has little effect on the corrosion rate.

Oxygen gas inhibits the corrosion of uranium by water.HP Pavilion DV4Z-1200 CTO Battery

Nuclear waste

Spent uranium dioxide fuel is very insoluble in water, it is likely to release uranium (and fission products) even more slowly than borosilicate glass when in contact with water.HP Pavilion DV4 Battery

Note that while the vast majority of the uranium is removed by PUREX nuclear reprocessing, a small amount of uranium is left in the raffinate from the first cycle of the PUREX process. In addition because of the decay of the transplutonium minor actinides and the residual plutonium in the waste the concentration of uranium will increase on the waste.HP Pavilion DV4-1000 Battery

This will occur on a time scale of hundreds and thousands of years.

Health effects

Soluble uranium salts are toxic, though less so than those of other heavy metals such as lead or mercury.

HP Pavilion DV4-1000EA Battery

The organ which is most affected is the kidney. Soluble uranium salts are readily excreted in the urine, although some accumulation in the kidneys does occur in the case of chronic exposure. The World Health Organization has established a daily "tolerated intake" of soluble uranium salts for the general public of 0.5 ?g/kg body weight (or 35 ?g for a 70 kg adult): exposure at this level is not thought to lead to any significant kidney damage.

HP Pavilion DV4-1000ET Battery

The antidote for uranium in humans is bicarbonate, this is used because uranium (VI) forms complexes with carbonate. An alternative is to use tiron (sodium 4,5-dihydroxybenzene-1,3-disulfonate).

The use of depleted uranium (DU) in munitions is controversial because of questions about potential long-term health effects.

HP Pavilion DV4-1001 Battery

Normal functioning of the kidney, brain, liver, heart, and numerous other systems can be affected by uranium exposure, because uranium is a toxic metal.[2] The aerosol produced during impact and combustion of depleted uranium munitions can potentially contaminate wide areas around the impact sites leading to possible inhalation by human beings.HP Pavilion DV4-1001AX Battery

During a three week period of conflict in 2003 in Iraq, 1,000 to 2,000 tonnes of DU munitions were used.

The actual acute and chronic toxicity of DU is also a point of medical controversy. Multiple studies using cultured cells and laboratory rodents suggest the possibility of leukemogenic, genetic, reproductive, and neurological effects from chronic exposure.HP Pavilion DV4-1001TU Battery

A 2005 epidemiology review concluded: "In aggregate the human epidemiological evidence is consistent with increased risk of birth defects in offspring of persons exposed to DU." The World Health Organization, the directing and coordinating authority for health within the United Nations which is responsible for setting health research norms and standards, providing technical support to countries and monitoring and assessing health trends,[

HP Pavilion DV4-1001TX Battery

states that no risk of reproductive, developmental, or carcinogenic effects have been reported in humans due to DU exposure.[22]This report has been criticized by Dr. Keith Baverstock for not including possible long term effects of DU on human body.HP Pavilion DV4-1002 Battery

Birth defects

Most scientific studies have found no link between uranium and birth defects, but some claim statistical correlations between soldiers exposed to DU, and those who were not, concerning reproductive abnormalities.

One study concluded that epidemiological evidence is consistent with an increased risk of birth defects in the offspring of persons exposed to DU.HP Pavilion DV4-1002AX Battery

Environmental groups and others have expressed concern about the health effects of depleted uranium, and there is some debate over the matter. Some people have raised concerns about the use of this material, particularly in munitions, because of its mutagenicity, teratogenicity in mice, and neurotoxicity,  and its suspected carcinogenic potential. HP Pavilion DV4-1002TU Battery

Additional concerns address unexploded DU munitions leeching into groundwater over time.

Several sources have attributed the increase in the rate of birth defects in the children of Gulf War veterans and in Iraqis to depleted uranium inhalation exposure, HP Pavilion DV4-1002TX Battery

A 2001 study of 15,000 February 1991 U.S. Gulf War combat veterans and 15,000 control veterans found that the Gulf War veterans were 1.8 (fathers) to 2.8 (mothers) times more likely to have children with birth defects. In a study of UK troops, "Overall, the risk of any malformation among pregnancies reported by men was 50% higher in Gulf War Veterans (GWV) compared with Non-GWVs". HP Pavilion DV4-1003AX Battery

The conclusion of the study stated "We found no evidence for a link between paternal deployment to the Gulf war and increased risk of stillbirth, chromosomal malformations, or congenital syndromes. Associations were found between fathers' service in the Gulf war and increased risk of miscarriage and less well-defined malformationsHP Pavilion DV4-1003TU Battery

, but these findings need to be interpreted with caution as such outcomes are susceptible to recall bias. The finding of a possible relationship with renal anomalies requires further investigation. There was no evidence of an association between risk of miscarriage and mothers' service in the gulf."HP Pavilion DV4-1003TX Battery

Animals

It has been reported that uranium has caused reproductive defects, and other health problems in rodents, frogs and other animals. Uranium was shown to have cytotoxic, genotoxic and carcinogenic effects in animal studies.It has been shown in rodents and frogs that water soluble forms of uranium areteratogenic.HP Pavilion DV4-1004AX Battery

Bacterial biochemistry

It has been shown that bacteria can reduce and fix uranium in soils.[ These bacterium change soluble U(VI) into the highly insoluble complex forming U(IV) ion, hence stopping chemical leaching.HP Pavilion DV4-1004TU Battery

Behaviour in soil

It has been suggested that it is possible to form a reactive barrier by adding something to the soil which will cause the uranium to become fixed. One method of doing this is to use a mineral (apatite)  while a second method is to add a food substance such as acetate to the soil.HP Pavilion DV4-1004TX Battery

This will enable bacteria to reduce the uranium (VI) to uranium (IV) which is much less soluble. In peat-like soils the uranium will tend to bind to the humic acids, this tends to fix the uranium in the soil.HP Pavilion DV4-1005TX Battery,HP Pavilion DV4-1006TX Battery,HP Pavilion DV4-1007TX Battery,HP Pavilion DV4-1008TX Battery,HP Pavilion DV4-1009TX Battery,HP Pavilion DV4-1010TX Battery,HP Pavilion DV4-1011TX Battery

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