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6 janvier 2012 5 06 /01 /janvier /2012 03:45

Polonium is a chemical element with the symbol Po and atomic number 84, discovered in 1898 byMarie Sk?odowska-Curieand Pierre Curie. A rare and highly radioactive element, polonium is chemically similar to bismuth[1]and tellurium, and it occurs in uranium ores. Polonium has been studied for possible use in heating spacecraft. Sony VAIO VPCEB1LFX Battery

As it is unstable, all isotopes of polonium are radioactive. There is disagreement as to whether polonium is a post-transition metalor metalloid.[2][3]

Characteristics

Polonium has 33 known isotopes, all of which are radioactive. They have atomic masses that range from 188 to 220 u. 210Po (half-life 138.4 days) is the most widely available. Sony VAIO VPCEB1LFX/BI Battery

209Po (half-life 103 years) and 208Po (half-life 2.9 years) can be made through the alpha, proton, or deuteron bombardment of lead or bismuth in a cyclotron.

210Po is an alpha emitter that has a half-life of 138.4 days; it decays directly to its stable daughter isotope, 206Pb. A milligram of 210Po emits about as many alpha particles per second as 4.5 grams of 226Ra. Sony VAIO VPCEB1LFX/WI Battery

A few curies (1 curie equals 37gigabecquerels, 1 Ci = 37 GBq) of 210Po emit a blue glow which is caused by excitation of surrounding air. A single gram of210Po generates 140 watts of power.[4] Because it emits many alpha particles, which are stopped within a very short distance in dense media and release their energy, 210Po has been used as a lightweight heat source to power thermoelectric cells inartificial satellites; Sony VAIO VPCEB1MFX Battery

for instance, 210Po heat source was also used in each of the Lunokhod rovers deployed on the surface of the Moon, to keep their internal components warm during the lunar nights.[5] Some anti-static brushes contain up to 500 microcuries (20 MBq) of 210Po as a source of charged particles for neutralizing static electricity in materials like photographic film.[6] Sony VAIO VPCEB1MFX/BI Battery

About one in 100,000 alpha emissions causes an excitation in the nucleus which then results in the emission of a gamma ray.[7]Because of its relatively high rate of alpha emissions, only about 1 in 100,000 result in releasing a gamma ray. But it is the alpha particles, not the side effect of an occasional gamma ray, that results in 210Po decay. Sony VAIO VPCEB1NFX Battery

Low gamma output renders gamma detection nearly impossible, with any emitted gamma nearly indistinguishable from background radiation. At 4.001 u, the alpha particle is too massive to penetrate most barriers, including intact human epidermis. If the skin is broken however, or the alpha emitter is ingested or inhaled, the high charge on the alpha particle will result in severe cellular damage. Sony VAIO VPCEB1NFX/B Battery

The high alpha decay of polonium renders alpha detection as the preferred method of quantifying this isotope in the laboratory.

Solid state form

Polonium is a radioactive element that exists in two metallic allotropes. The alpha form is the only known example of a simple cubiccrystal structure in a single atom basis, with an edge length of 335.2 picometres; the beta form is rhombohedral.[8][9] Sony VAIO VPCEB1NFX/L Battery

The structure of polonium has been characterized by X-ray diffraction [10][11] and electron diffraction.[12]

210Po (in common with 238Pu) has the ability to become airborne with ease: if a sample is heated in air to 55 °C (131 °F), 50% of it is vaporized in 45 hours, even though the melting point of polonium is 254 °C (489 °F) and its boiling point is 962 °C (1763 °F).[13][14]Sony VAIO VPCEB1NFX/P Battery

More than one hypothesis exists for how polonium does this; one suggestion is that small clusters of polonium atoms are spalled offby the alpha decay.

Chemistry

The chemistry of polonium is similar to that of tellurium and bismuth. Polonium dissolves readily in dilute acids, but is only slightly soluble in alkalis. Sony VAIO VPCEB1NFX/W Battery

The hydrogen compound PoH2 is liquid at room temperature (melting point ?36.1°C, boiling point35.3°C). Halides of the structure PoX2, PoX4 and PoX6 are known. The two oxides PoO2and PoO3 are the products of oxidation of polonium.[15]

It has been reported that some microbes can methylate polonium by the action of methylcobalamin.[16][17] Sony VAIO VPCEB1PFX Battery

This is similar to the way in which mercury, selenium and tellurium are methylated in living things to create organometallic compounds. As a result when considering the biochemistry of polonium one should consider the possibility that the polonium will follow the same biochemical pathways as selenium and tellurium. Sony VAIO VPCEB1PFX/B Battery

Also tentatively called "Radium F", polonium was discovered by Marie Sk?odowska-Curie and her husband Pierre Curie in 1898[18]and was named after Marie Curie's native land of Poland (Latin: Polonia)[19][20] Poland at the time was under Russian, Prussian, and Austrian partition, and did not exist as an independent country. Sony VAIO VPCEB1QGX Battery

It was Curie's hope that naming the element after her native land would publicize its lack of independence. Polonium may be the first element named to highlight a political controversy.[21]

This element was the first one discovered by the Curies while they were investigating the cause of pitchblende radioactivity. Sony VAIO VPCEB1QGX/BI Battery

The pitchblende, after removal of the radioactive elements uranium and thorium, was more radioactive than both the uranium and thorium put together. This spurred the Curies on to find additional radioactive elements. The Curies first separated out polonium from the pitchblende, and then within a few years, also isolated radium. Sony VAIO VPCEB1RGX Battery

Because of the small quantities present in nature, isolation of polonium from natural sources is complicated. The largest ever isolated batch from 37 tonnes of residues from radium production yielded only 40 Ci of polonium-210.[22] Sony VAIO VPCEB1RGX/BI Battery

Gamma counting

By means of radiometric methods such as gamma spectroscopy (or a method using a chemical separation followed by an activitymeasurement with a non-energy-dispersive counter), it is possible to measure the concentrations of radioisotopes and to distinguish one from another. Sony VAIO VPCEB20 Battery

In practice, background noise would be present and depending on the detector, the line width would be larger which would make it harder to identify and measure the isotope. In biological/medical work it is common to use the natural 40K present in all tissues/body fluids as a check of the equipment and as an internal standard. Sony VAIO VPCEC20 Battery

Alpha counting

The best way to test for (and measure) many alpha emitters is to use alpha-particle spectroscopy as it is common to place a drop of the test solution on a metal disk which is then dried out to give a uniform coating on the disk. This is then used as the test sample.Sony VAIO VPCEE20 Battery

If the thickness of the layer formed on the disk is too thick then the lines of the spectrum are broadened, this is because some of the energy of the alpha particlesis lost during their movement through the layer of active material. An alternative method is to use internal liquid scintillation where the sample is mixed with a scintillation cocktail. Sony VAIO VPCEF20 Battery

When the light emitted is then counted, some machines will record the amount of light energy per radioactive decay event. Due to the imperfections of the liquid scintillation method (such as a failure for all the photons to be detected, cloudy or coloured samples can be difficult to count) and the fact that random quenching can reduce the number of photons generated per radioactive decay it is possible to get a broadening of the alpha spectra obtained through liquid scintillation. Sony VAIO VPCF112FX/B Battery

It is likely that these liquid scintillation spectra will be subject to a Gaussian broadeningrather than the distortion exhibited when the layer of active material on a disk is too thick.

A third energy dispersive method for counting alpha particles is to use a semiconductor detector. Sony VAIO VPCF115FG/B Battery

From left to right the peaks are due to 209Po, 210Po, 239Pu and 241Am. The fact that isotopes such as 239Pu and 241Am have more than one alpha line indicates that the nucleus has the ability to be in different discrete energy levels (like a molecule can).

Occurrence and production

Polonium is a very rare element in nature because of the short half-life of all its isotopes. Sony VAIO VPCF116FGBI Battery

It is found in uranium ores at about 100 micrograms per metric ton (1 part in 1010), which is approximately 0.2% of the abundance of radium. The amounts in the Earth's crust are not harmful. Polonium has been found in tobacco smoke from tobacco leaves grown with phosphatefertilizers.[23][24][25] Sony VAIO VPCF117FJ/W Battery

Synthesis by (n,?) reaction

In 1934 an experiment showed that when natural 209Bi is bombarded with neutrons, 210Bi is created, which then decays to 210Po via ? decay. The final purification is done pyrochemically followed by liquid-liquid extraction techniques.[26] Sony VAIO VPCF117HG/BI Battery

Polonium may now be made in milligram amounts in this procedure which uses high neutron fluxes found in nuclear reactors. Only about 100 grams are produced each year, practically all of it in Russia, making polonium exceedingly rare.[27][28] Sony VAIO VPCF118FJ/W Battery

Synthesis by (p,n) and (p,2n) reactions

It has been found that the longer-lived isotopes of polonium can be formed by proton bombardment of bismuth using a cyclotron. Other more neutron rich isotopes can be formed by the irradiation of platinum with carbon nuclei.[29] Sony VAIO VPCF119FC Battery

Applications

When it is mixed or alloyed with beryllium, polonium can be a neutron source: beryllium releases a neutron upon absorption of an alpha particle that is supplied by210Po. It has been used in this capacity as a neutron trigger or initiator for nuclear weapons.[30]Other uses include the following. Sony VAIO VPCF119FC/BI Battery

Devices that eliminate static charges in textile mills and other places.[31] However, beta particle sources are more commonly used and are less dangerous. A non-radioactive alternative is to use a high-voltage DC power supply to ionise air positively or negatively as required.[32] Sony VAIO VPCF119FJ/BI Battery

  • 210Po can be used as an atomic heat source to power radioisotope thermoelectric generators via thermoelectricmaterials.[33]
  • Because of its very high toxicity, polonium can be used as a poison (see, for example, Alexander Litvinenko poisoning).

Polonium-containing anti-static brushes are used to remove dust on photographic film.[34] Sony VAIO VPCF11AFJ Battery

Overview

By mass, polonium-210 is around 250,000 times more toxic than hydrogen cyanide (the actual LD50 for 210Po is less than 1 microgram for an average adult (see below) compared with about 250 milligrams for hydrogen cyanide[35]). The main hazard is its intense radioactivity (as an alpha emitter), which makes it very difficult to handle safely: Sony VAIO VPCF11AGJ Battery

one gram of Po will self-heat to a temperature of around 500 °C (932 °F).[4] Even in microgram amounts, handling 210Po is extremely dangerous, requiring specialized equipment (a.o., alpha glove box under depression and equipped with high performance filters), adequate monitoring, and strict handling procedures to avoid any contamination. Sony VAIO VPCF11AHJ Battery

Alpha particles emitted by polonium will damage organic tissue easily if polonium is ingested, inhaled, or absorbed, although they do not penetrate the epidermis and hence are not hazardous as long as the alpha particles remain outside of the body. Meanwhile, bearing chemically resistant and "intact" gloves is a mandatory precautions to avoid transcutaneous diffusion of polonium directly through the skin. Sony VAIO VPCF11JFX/B Battery

Polonium delivered in concentrated nitric acid can easily diffuse through inadequate gloves (e.g., latex gloves) or the acid may damage the gloves.

Acute effects

The median lethal dose (LD50) for acute radiation exposure is generally about 4.5 Sv.[36] The committed effective dose equivalent210Po is 0.51 µSv/Bq if ingested, and 2.5 µSv/Bq if inhaled.[37] Sony VAIO VPCF11M1E Battery

Since 210Po has an activity of 166 TBq per gram (4,500 Ci/g)[37](1 gram produces 166×1012 decays per second), a fatal 4.5 Sv (J/kg) dose can be caused by ingesting 8.8 MBq (238 microcuries, µCi), about 50 nanograms (ng), or inhaling 1.8 MBq (48 µCi), about 10 ng. One gram of 210Po could thus in theory poison 20 million people of whom 10 million would die. Sony VAIO VPCF11M1E/H Battery

The actual toxicity of 210Po is lower than these estimates, because radiation exposure that is spread out over several weeks (the biological half-life of polonium in humans is 30 to 50 days[38]) is somewhat less damaging than an instantaneous dose. It has been estimated that a median lethal dose of 210Po is 0.015 GBq (0.4 mCi), or 0.089 micrograms, still an extremely small amount.[39][40] Sony VAIO VPCF11MFX/B Battery

Long term (chronic) effects

In addition to the acute effects, radiation exposure (both internal and external) carries a long-term risk of death from cancer of 5–10% per Sv.[36] The general population is exposed to small amounts of polonium as a radon daughter in indoor air; Sony VAIO VPCF11S1E Battery

the isotopes 214Po and 218Po are thought to cause the majority[41] of the estimated 15,000-22,000 lung cancer deaths in the US every year that have been attributed to indoor radon.[42] Tobacco smoking causes additional exposure to polonium.[43]

Regulatory exposure limits

The maximum allowable body burden for ingested 210Po is only 1.1 kBq (30 nCi), which is equivalent to a particle massing only 6.8 picograms. Sony VAIO VPCF11S1E/B Battery

The maximum permissible workplace concentration of airborne 210Po is about 10 Bq/m3 (3 × 10?10 µCi/cm3).[44] The target organs for polonium in humans are the spleen and liver.[45] As the spleen (150 g) and the liver (1.3 to 3 kg) are much smaller than the rest of the body, if the polonium is concentrated in these vital organs, it is a greater threat to life than the dose which would be suffered (on average) by the whole body if it were spread evenly throughout the body, in the same way as caesium or tritium (as T2O). Sony VAIO VPCF11Z1E Battery

210Po is widely used in industry, and readily available with little regulation or restriction. In the US, a tracking system run by the Nuclear Regulatory Commission will be implemented in 2007 to register purchases of more than 16 curies (590 GBq) of polonium 210 (enough to make up 5,000 lethal doses). Sony VAIO VPCF11Z1E/BI Battery

The IAEA "is said to be considering tighter regulations... There is talk that it might tighten the polonium reporting requirement by a factor of 10, to 1.6 curies (59 GBq)."[46]

Famous poisoning cases

Notably, the murder of Alexander Litvinenko, a Russian dissident, in 2006 was announced as due to 210Po poisoning[47][48](see Alexander Litvinenko poisoning). Sony VAIO VPCF11ZHJ Battery

According to Prof. Nick Priest of Middlesex University, an environmental toxicologist and radiation expert, speaking on Sky News on December 2, Litvinenko was probably the first person ever to die of the acute ?-radiation effects of 210Po.[49]

It has also been suggested that Irène Joliot-Curie was the first person ever to die from the radiation effects of polonium (due to a single intake) in 1956.[50] Sony VAIO VPCF127HGBI Battery

She was accidentally exposed to polonium in 1946 when a sealed capsule of the element exploded on her laboratory bench. A decade later, on 17 March 1956, she died in Paris from leukemia which may have been caused by that exposure.

According to the book The Bomb in the Basement, several death cases in Israel during 1957-1969 were caused by 210Po.[51] Sony VAIO VPCF137HG/BI Battery

A leak was discovered at a Weizmann Institutelaboratory in 1957. Traces of 210Po were found on the hands of professor Dror Sadeh, a physicist who researched radioactive materials. Medical tests indicated no harm, but the tests did not include bone marrow. Sadeh died from cancer. Sony VAIO VPC-P111KX/B Battery

One of his students died of leukemia, and two colleagues died after a few years, both from cancer. The issue was investigated secretly, and there was never any formal admission that a connection between the leak and the deaths had existed.[52]

Treatment

It has been suggested that chelation agents such as British Anti-Lewisite (dimercaprol) can be used to decontaminate humans.[53]Sony VAIO VPC-P111KX/D Battery

In one experiment, rats were given a fatal dose of 1.45 MBq/kg (8.7 ng/kg) of 210Po; all untreated rats were dead after 44 days, but 90% of the rats treated with the chelation agent HOEtTTC remained alive after 5 months.[54] Sony VAIO VPC-P111KX/G Battery

Commercial products containing polonium

Potentially lethal amounts of polonium are present in anti-static brushes sold to photographers.[55] In USA, the devices with no more than 500 µCi of (sealed) 210Po per unit can be bought in any amount under a "general license",[56] which means that a buyer need not be registered by any authorities. Sony VAIO VPC-P111KX/P Battery

Tiny amounts of such radioisotopes are sometimes used in the laboratory and for teaching purposes—typically of the order of 4–40 kBq (0.1–1.0 µCi), in the form of sealed sources, with the polonium deposited on a substrate or in a resin or polymer matrix—are often exempt from licensing by the NRC and similar authorities as they are not considered hazardous. Sony VAIO VPC-P111KX/W Battery

Small amounts of 210Po are manufactured for sale to the public in the United States as 'needle sources' for laboratory experimentation, and are retailed by scientific supply companies. The actual polonium is a layer of plating which in turn is plated with a material such as gold. Sony VAIO VPC-P112KX/B Battery

This allows the alpha radiation (used in experiments such as cloud chambers) while preventing the polonium from being released and presenting a toxic hazard. According to United Nuclear, they typically sell between four and eight sources per year.[57][58]

Occurrence in humans and the biosphere

The natural occurrence of polonium-210 is widespread in the biosphere, including in human tissues. Sony VAIO VPC-P112KX/D Battery

This is a consequence of a mechanistic chain involving naturalradioactive fallout. Radium-226 in the Earth's crust decays with the production of radon-220, some of which, during its 3.6-day half-life, diffuses into the atmosphere. Here it decays with the production of polonium-210, much of which, during its 138-day half-life, is washed back down to the Earth's surface, thus entering the biosphere.[59][60][61] Sony VAIO VPC-P112KX/G Battery

As early as the 1920s Lacassagne, using polonium provided by his colleague Marie Curie, showed that the element has a very specific pattern of uptake in rabbit tissues, with high concentrations particularly in liver, kidney and testes.[62] More recent evidence suggests that this behaviour results from polonium substituting for sulphur in S-containing amino-acids or related molecules[63][64] Sony VAIO VPC-P112KX/P Battery

and that similar patterns of distribution occur in human tissues.[65] Polonium is indeed an element naturally present in all humans, contributing appreciably to natural background dose, with wide geographical and cultural variations, and particularly high levels in arctic residents, for example.[66] Sony VAIO VPC-P112KX/W Battery

Tobacco

The presence of polonium in tobacco smoke has been known since the early 1960s.[67][68] Some of the world's biggest tobacco firms researched ways to remove the substance—to no avail—over a 40-year period but never published the results.[25] Sony VAIO VPCP113KX/B Battery

Radioactive polonium-210 contained in phosphate fertilizers is absorbed by the roots of plants (such as tobacco) and stored in its tissues.[69][70][71] Tobacco plants fertilized by rock phosphates contain polonium-210, which emits alpha radiation estimated to cause about 11,700 lung cancer deaths annually worldwide. Sony VAIO VPC-P113KX/B Battery

Astatine is a radioactive chemical element with the symbol At and atomic number 85. It occurs on the Earth only as the result of decay of heavier elements, and decays away rapidly, so much less is known about this element than its upper neighbors in the periodic table. Sony VAIO VPCP113KX/D Battery

However, research has shown this element follows periodic trends, being the heaviest known halogen, with melting and boiling points being higher than those of lighter halogens.

Astatine was first produced by Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè in the University of California, Berkeley in 1940. Sony VAIO VPC-P113KX/D Battery

Three years later, it was found in nature; however, with an estimated amount of less than 28 grams (1 oz) at given time, astatine is the least abundant element in Earth's crust among non-transuranium elements. Among astatine isotopes, six (with mass numbers 214 to 219) are present in nature, but the most stable astatine-210 is not. Sony VAIO VPCP113KX/G Battery

The second longest-living astatine-211 is the only one to find a commercial use, being useful as an alpha emitter in medicine; however, only extremely small quantities are used, and in larger ones it is very hazardous, as it is intensely radioactive.

Until recently most of the chemical characteristics of astatine were inferred from comparison with other elements; however, important studies have already been done. Sony VAIO VPC-P113KX/G Battery

The main difference between astatine and iodine is that the HAt molecule is chemically a hydride rather than a halide; however, in a fashion similar to the lighter halogens, it is known to form ionic astatides with metals. Bonds to nonmetals result in positive oxidation states, with +1 best portrayed by monohalides and their derivatives, while the higher are characterized by bond to oxygen and carbon. Sony VAIO VPCP113KX/P Battery

Attempts to synthesize astatine fluoride have been met with failure.

General characteristics

Astatine is a highly radioactive element: its isotopes have their half-lives under half a day, decaying into those ofbismuth, polonium, radon, or other astatine isotopes. Among the first 103 elements, only francium and nobelium, the latter of which does not occur in nature, are less stable than astatine.[1] Sony VAIO VPC-P113KX/P Battery

Astatine is a halogen,[2] though it is more metallic than iodine. Research into astatine is limited by the very small quantities available, which is a consequence of its extremely short half-life;[3] however, it has been shown that astatine suits most periodic trends very well. Like other halogens, it is composed of diatomic At2 molecules on standard conditions.[4] Sony VAIO VPCP113KX/W Battery

Its melting and boiling point are estimated about 300 °C (575 °F) and 370 °C (700 °F), respectively. This allows astatine to fit well the trend in increase in melting and boiling points with the atomic number among halogens.[3]

The element is often cited to have the electronegativity of 2.2 (Pauling scale), as this is stated in Pauling's work,[5] Sony VAIO VPC-P113KX/W Battery

lower than that of iodine (2.5 in the original work and 2.66 now[6]) and same as hydrogen; the experiments have shown that the actual astatine electronegativity is slightly below that of hydrogen[7] (see below). It sublimes more readily than iodine, with lower vapor pressure; it also dissolves in water.[3] Sony VAIO VPC-P114KX/B Battery

There are 32 known isotopes of astatine, with atomic masses of of 191 and 193–223.[1] No stable or at least long-lived astatine isotope is known, and no such an isotope is expected to exist.[8]

Alpha decay energy follows the same trend as other heavy elements.[8] Lighter astatine isotopes have quite high energies of alpha decay, which get lower as the nuclei get heavier. Sony VAIO VPC-P114KX/D Battery

But astatine-211, the nucleus with 126 neutrons (126 is a magic number and corresponds to a filled neutron shell) has a significantly higher energy than the previous one. Despite having a similar half-life time with the previous isotope (8.1 hours for 210At and 7.2 hours for 211At), the alpha decay probability is way higher for the latter: 41.81% against only 0.18%.[1][note 2] Sony VAIO VPC-P114KX/G Battery

The two following isotopes release even more energy, with astatine-213 releasing the highest amount of energy of all astatine isotopes. For this reason, it is the shortest-lived astatine isotope.[8] Even though heavier astatine isotopes release less energy, no long-lived astatine isotope exists; this happens due to increasing role of beta decay.[8] Sony VAIO VPC-P114KX/P Battery

This decay mode is especially important for astatine: as early as 1950, it has been postulated that the element has no beta-stable isotopes (i.e. those that do not beta decay at all).[9] The mode has already been found for all astatine isotopes, except for 213At, 214At, 215At, 216mAt, and 217At.[1] Sony VAIO VPC-P114KX/W Battery

The most stable of them is astatine-210, which (as it has been mentioned) has a half-life of 8.1 hours. This isotope's primary decay mode is beta decay to a relatively long-lived (compared to astatine isotopes) alpha emitter, polonium-210. In total, only five isotopes have half-lives exceeding one hour, namely those with mass numbers between 207 and 211. Sony VAIO VPCP115JC Battery

The least stable ground state isotope is astatine-213, with a half-life of 125 ns. It alpha decays to the extremely long-lived (in practice, stable) bismuth-209.[1]

In nature, six astatine isotopes are present: astatine-214 to -219; they decay rapidly by primarily alpha decay, but are rapidly recreated by the decay of heavier elements as well. Sony VAIO VPCP115JC/B Battery

Astatine-215, -217, -218, and -219 are present in the major decay chains,[10]while astatine-214 and -216 (as well as astatine-215) occur as result of decay of natural protactinium.[8]

Astatine has 23 nuclear isomers (nuclei with of one or more nucleons (protons or neutrons) excited. Sony VAIO VPCP115JC/D Battery

A nuclear isomer may also be called a "meta state," which means the system has more internal energy than the "ground state" (the state with the lowest possible internal energy), making the former likely to decay into it, not always one isomer for one isotope. The most stable of them is astatine-202m1, which has a half-life of about 3 minutes; Sony VAIO VPCP115JC/G Battery

this is longer than those of all ground states, except for isotopes with mass numbers 203–211 and 220. The least stable one is astatine-214m1; its half-life of 265 ns is shorter than those of all ground states, except for astatine-213.[1]

In 1869, Dmitri Mendeleev published his periodic table. The space under iodine was empty; Sony VAIO VPCP115JC/P Battery

after Niels Bohr established the physical basis of the classification of chemical elements, it was suggested that there could be the fifth halogen. Before officially recognized discovery, it was called "eka-iodine," from Sanskrit eka, "one," to imply it was one space under iodine (in the same manner as eka-silicon, eka-boron, and others).[11] Scientists tried to find it in nature, which led to erroneous discoveries.[12] Sony VAIO VPCP115JC/W Battery

The first claimed discovery of eka-iodine was made by Fred Allison and associates at the Alabama Polytechnic Institute (now Auburn University) in 1931; the discoverers named element 85 "alabamine" and assigned it the symbol Ab, which were used for a few years.[13][14][15] Sony VAIO VPCP115KG Battery

In 1934, however, H. G. MacPherson of University of California, Berkeleydisproved the effectiveness of Allison's device and the validity of this discovery.[16] This erroneous discovery was followed by another claim in 1937, by the chemist Rajendralal De. Working in Dhaka, British India (now Bangladesh), he chose the name "dakin" for element 85, which he claimed to have isolated as the thorium series equivalent of Radium F (polonium-210) in the radium series. Sony VAIO VPCP116KG Battery

The properties he reported for dakin do not correspond to those of astatine, and its identity is not known.[17]

In 1940, the Swiss chemist Walter Minder announced the discovery of element 85 as the product of the beta decay of Radium A (polonium-218), choosing the name "helvetium" (from Helvetia, "Switzerland").Sony VAIO VPC-P116KX/B Battery

However, Berta Karlik and Traude Bernert were unsuccessful in reproducing his experiments, attributing the results to contamination of his radon stream.[18] (Radon-222 is the parent isotope of polonium-218.) In 1942, Minder, in collaboration with the English scientist Alice Leigh-Smith, announced the discovery of another isotope of element 85, presumed to be the product of Thorium A (polonium-216) beta decay. Sony VAIO VPC-P116KX/D Battery

They named this substance "anglo-helvetium,"[19] but Karlik and Bernert were again unable to reproduce these results.[20]

In 1940, Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè finally isolated the element at the University of California, Berkeley. Sony VAIO VPC-P116KX/G Battery

Instead of searching for the element in nature, the scientists actually created it by bombarding bismuth with alpha particles.[21] The name "astatine" comes from the Greek word ??????? astatos, meaning "unstable," due to the created isotope's propensity for radioactive decay (later, all isotopes of the element were shown to be unstable),[21] and the ending "-ine," found in the names of the four previously discovered halogens. Sony VAIO VPC-P116KX/P Battery

Three years later, astatine was found as a product of natural decay chains by Karlik and Bernert.[22][23] Since then, astatine has been found in three out of the four natural decay chains.[24]

Natural occurrence

Astatine is the rarest naturally occurring element among those that do not belong to the transuranium elements series, with the total amount in Earth's crust estimated to be less than 28 grams (1 oz) at given time.[25] Sony VAIO VPC-P116KX/W Battery

Astatine present at the formation of the Earth has long since decayed; natural astatine at present has formed through the decay of heavier elements. Previously thought to be the rarest element occurring on the Earth, astatine has lost this status to berkelium, atoms of which can be produced by neutron capture reactions and beta decay in very highly concentrated uranium-bearing deposits.[26] Sony VAIO VPCP118JC Battery

Six astatine isotopes occur naturally: these are astatine-214 to -219.[27] Because of their short half-lives, they are found only in traceamounts.[10] Although astatine is found on the Earth, there is no data of astatine occurrence in stars.[28]

Neptunium series, showing the decay products, including astatine-217, formed from neptunium-237. Sony VAIO VPCP118JC/B Battery

Four out of these isotopes (215At, 217At, 218At, and 219At) are found there due to their production in major natural decay chains. The father isotope of astatine-219, francium-223, alpha decays with a probability of only 0.006%, making this astatine isotope extremely rare even compared to other astatine isotopes, although its half-life is the longest of the natural astatine isotopes at 56 seconds.[1] Sony VAIO VPCP118JC/P Battery

This rare isotope decays to polonium-215, which itself beta decays to astatine-215 with an even smaller probability of 0.00023%; for this reason, the Americas to a depth of sixteen kilometers (10 mi) contains only a trillion astatine-215 atoms at given time.[29]Astatine-218 is found in nature as result of polonium-218 beta decay; Sony VAIO VPCP118JC/W Battery

like francium-223 and polonium-215, decay to an astatine isotope is not the primary decay mode.[10] Therefore, most Earth's astatine is astatine-217, whose father (francium-221) decays exclusively to this nuclide; its fathers, grandfathers, so on, decay to exclusively to one exact nuclide, to make only one possible way for the starting nuclide in the neptunium series, neptunium-237, to decay to astatine-217.[10] Sony VAIO VPC-P118KX/B Battery

The other remaining isotopes (214At and 216At, as well as 215At) are found as the result of triple alpha decay of the naturally present protactinium isotopes: 226Pa, 227Pa, and 228Pa.[27] These, however, are extremely rare, so they are often even not cited as natural astatine isotopes.[3][30]

Chemical reactivity and compounds

Astatine is the least reactive of the halogens, being less reactive than iodine;[31] Sony VAIO VPC-P118KX/D Battery

however, multiple compounds of astatine have been synthesized in microscopic amounts and studied as intensively as possible before their inevitable radioactive disintegration. The reactions are normally tested with dilute solutions of astatine mixed with larger amounts of iodine. The iodine acts as a carrier, ensuring that there is sufficient material for laboratory techniques such as filtration and precipitation to work.[20][32] Sony VAIO VPC-P118KX/G Battery

The most common compound of the element is hydrogen astatide.[3] The hydrogen astatide molecule has been calculated to have a dipole moment of 0.06 debyes, with hydrogen carrying the partial negative charge. Because astatine has a lower electronegativity when compared to hydrogen (unlike the other halogens), the molecule should more properly be called astatine monohydride;[7]Sony VAIO VPC-P118KX/P Battery

this reversal of polarity partially explains its lower stability compared to the other hydrogen halides. As it is easily oxidized, it is precipitated in aqueous nitric acid/silver(I) solution, forming silver(I) astatide, AgAt.[3]

Astatine is known to react with its lighter homologues iodine, bromine, and chlorine in the vapor state; Sony VAIO VPC-P118KX/W Battery

this reaction produces diatomic interhalogen compounds, with formulas AtI, AtBr, and AtCl.[2] The first two compounds may also be produced in water: astatine reacts with iodine/iodide solution to form AtI, whereas AtBr requires, aside from astatine, a iodine/iodine monobromide/bromide solution. Sony VAIO VPCP119JC Battery

The excess of iodides or bromides may lead to AtBr?
2 and AtI?
2 ions;[2] in a chloride solution, they may turn to species like AtCl?
2 or AtBrCl? via equilibrium.[33] No report of gas phase AtCl preparation has been shown, but oxidation of the element with dichromate (in nitric acid solution) showed that adding chloride turned the astatine into a molecule, either AtCl or AtOCl; Sony VAIO VPCP119JC/BI Battery

similarly, AtOCl?
2 or AtCl?
2 may be produced.[2] In a plasma ion source mass spectrometer, similar ions [AtI]+, [AtBr]+, and [AtCl]+ have been formed by introducing vapors of the lighter halogens to the helium-filled camera where astatine is situated, supporting the existence of stable neutral molecules in the plasma ion state.[2] Sony VAIO VPCP11S1E/B Battery

No astatine fluoride has not been discovered yet, and although its synthesis is thought to be possible, it may require a liquid halogen fluoride solvent; this has already been used for characterization of radon fluorides.[2]

The lower oxidation states are the starting point for astatine–oxygen bonds:[note 3] Sony VAIO VPC-P11S1E/B Battery

treating them with an oxygen-containing oxidizer leads to the formation of astatate ions, AtO?
3.[35] Further oxidation, e.g., by hypochlorite or electrochemical oxidation, was originally thought to form unstable astatine(VII), either as perastatic acid H5AtO6 (analogous to periodic acid) or perastatate AtO?
4;[3] however, this has never been confirmed.[36] Sony VAIO VPCP11S1E/D Battery

The intermediate states coprecipitate with several silver(I), thallium(I) or caesium oxysalts to form cationic astatine.[35] Astatine may also replace a hydrogen atom in benzene to form C6H5At, which may be oxidized to C6H5AtCl2 by chlorine; by treating this compound in an alkaline solution of hypochlorite, C6H5AtO2 may be produced.[35] Sony VAIO VPC-P11S1E/D Battery

Astatine may form bonds to the other chalcogens, such as S7At+ and At(CSN)?
2 with sulfur, a coordination selenocarbamide compound with selenium, and astatine–tellurium colloid with tellurium.[37]Additionally, astatine is known to bind to nitrogen,[38] lead,[39] and boron.[40] Sony VAIO VPCP11S1E/G Battery

Production

Astatine was first produced by bombarding bismuth with energetic alpha particles; this is still the major route used to create the relatively long-lived isotopes astatine-209 through -211. Astatine is only produced in microscopic quantities, with modern techniques allowing production runs of 2 terabecquerel (about 25 ?g).[42] Sony VAIO VPC-P11S1E/G Battery

The most important isotope is now astatine-211, being the only one to find a commercial use.[43] To produce the bismuth target, the metal is sputtered on a gold, copper, or aluminum surface, to form a 50–100 milligrams per centimeter squared bismuth layer (or, alternatively, bismuth oxide is pressed into a copper plate).[44] Sony VAIO VPCP11S1E/P Battery

The target is kept under a chemically neutral nitrogenatmosphere[45] and is cooled with water to prevent the premature astatine vaporization.[44] In a particle accelerator,[46] such as a cyclotron, alpha particles are collided with bismuth. Even though bismuth is composed of only one isotope, bismuth-209, the reaction may occur in three possible ways, producing astatine-209, -210, and -211. Sony VAIO VPCP11S1E/W Battery

In order to eliminate the undesired nuclides, the maximum energy of the reaction is set to 28 MeV. The produced astatine is then easily separated, as astatine sets itself free and a stream of the element is collected as a deposit on a glass cold finger.[45]

Separation

The freshly formed astatine is separated either chemically or via distillation.[47] Sony VAIO VPC-P11S1E/W Battery

Astatine is very volatile, even though less than iodine (in acidic solutions).[note 4] Out of such solutions, the iron(II) sulfate/sulfuric acid solution seems the most promising: it may provide the distillation of 90% of the element (85% of the solution distilled).[49]

If dry distillation is used, the element (after its formation) must be separated from the target and the traces of other radioisotopes.Sony VAIO VPCP11Z9E/B Battery

The target is heated to 300–600 °C (575–1100 °F). The freshly vaporized astatine (in form of a stream) is collected as a deposit on a glass cold finger.[49] To eliminate the polonium content in astatine, it is re-distilled.

However, astatine is thought to react with bismuth to form non-volatile compounds. Sony VAIO VPCS111FM/S Battery

This is likely to cause the low astatine outcome (up to 15%).[49]. If the temperature is raised to 700–800 °C (1275–1450 °F), 80% of astatine may be distilled, which may be explained by suggesting that the non-volatile astatine compounds decompose; however, bismuth begins to vaporize as well. Mixing the methods, up to 30% of astatine may result.[49] Sony VAIO VPCS115EC Battery

Chemical routes

Astatine may be extracted from acidic aqueous solutions using organic solvents. The distribution factor varies on the solvent: the highest one is 200, found in 0.01 M nitric acid/benzene system (the second highest is 91, in 0.01 M nitric acid/tetrachloromethane system).[50] Sony VAIO VPCS115FG Battery

In general, it is similar to iodine in that it dissolves in benzene, carbon disulfide, and tetrachloromethane (therefore, they have high distribution factors in systems with water), but, unlike iodine, it cannot be extracted from alkaline solutions of organic solvents, as it reacts with bases and disproportionates.[3] Sony VAIO VPCS117GG Battery

Diatomic interhalogen molecules (with iodine and bromine) have smaller distribution factors than dihalogens (where both atoms are the same), as the former are dipoles.[50] Separation from other elements is done via extraction from hydrochloric acid/isopropyl ether solutions. Iron(III) hydroxide may be used to remove the further traces. Using 8 M HCl solutions, outcome may be as high as 90%.[50] Sony VAIO VPCS117GGB Battery

When astatine is introduced into negatively charged chloro complexes and dissolved in chloride or hydrogen chloride (at best at 5–8 M, because the complex decomposes on a lower chloride concentration), such a complex can be sorbed on a cation resin; the same is true for the astatine cation.[51] Sony VAIO VPCS118EC Battery

Such astatine is originally inserted into tellurium, which is (together with polonium impurity) later washer away with hydrochloric acid/chlorine solution. Astatine is later desorbed via chloric water. The cation resin is processed with nitric acid solution (with a small dichromate content), and the solution is introduced to it, and later to nitric acid solution (to remove the chloride ions). Thereafter, astatine is desorbed with nitric acid/dichromate solution.[52] Sony VAIO VPCS119FJ/B Battery

Astatine partially coprecipitates from weakly acidic solutions with several hydroxides, silver(I) and thallium(I) iodides, several sulfides of heavy metals, silver metal, and tellurium. This is likely to be caused by adsorbation on the surface of the precipitate.[53] The adsorbation is suppressed by increasing the acidity of the solution, washing off the precipitate with acetone, or adding iodine; the latter proves the adsorbation character of the astatine coprecipitation.[53] Sony VAIO VPCS119GC Battery

The most important such a reaction is the one with tellurium, which is catalyzed by reducers like tin dichloride; however, the reaction does not occu

r in alkaline solutions. The amount of astatine precipitated does not rely on tellurium amount, reaching 90% in concentrated HCl (this reaction also eliminates different impurities).[53]Astatide and astatate have been shown to coprecipitate with iodide and iodate; Sony VAIO VPCS11AFJ Battery

they, however, cannot be washed off with acetone.[54] These methods are used most typically only when astatine is the result of a different, way rarer reaction of bismuth, lead, or thorium with high-energy protons.[54] Since such reactions produces hundreds less times astatine than the one involving bismuth and alpha particles. Therefore, the method used of separation should be effective; this one has proved to be so.[54] Sony VAIO VPCS11AGJ Battery

Uses and precautions

The newly formed astatine-211 is important in prostate cancer treatment. Once produced, astatine should be used quickly, as astatine-211 decays with a half-life of 7.2 hours; this, however, is long enough to permit multi-step labeling strategies. Sony VAIO VPCS11AHJ Battery

Astatine-211 can be used for targeted alpha particle radiotherapy, since it decays either via alpha decay to bismuth-207, or via electronic capture to an extremely short-lived nuclide of polonium-211, which itself alpha decays.[55] A selection of astatine-containing organic compounds and their medical applications can be seen in the table below: Sony VAIO VPCS11AVJ Battery

Similarly to iodine, astatine is collected by the thyroid gland, even though to a lesser extent; however, it concentrates in the liver if released to the body if the form of a radiocolloid.[44] The principled behavior difference between astatine-211 and iodine-131 (a radioactive iodine isotope, also used in medicine) is that astatine does not destroy the neighboring parathyroid gland, as it does not emit beta particles: Sony VAIO VPCS11J7E/B Battery

an average alpha particle released by astatine-211 runs about 70 µm, while a beta particle emitted by iodine-131 runs about 2 mm.[44] Thanks to its short half-life and small particle run, it is preferable to be used in diagnosis of diseases (compared to iodine-131).[44] However, it attacks the thyroid gland much stronger, and the repetative injection of the nuclide caused tissue destruction in the gland, followed by dysplasia in rats and monkeys. Sony VAIO VPCS11M1E/W Battery

When lethal quantities are added, morphological changes in other tissues are not found, with the possible exception of the breasts.

Radon is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless noble gas, occurring naturally as the decay product of uranium or thorium. Sony VAIO VPCS11V9E Battery

Its most stable isotope, 222Rn, has a half-life of 3.8 days. Radon is one of the densest substances that remains a gas under normal conditions and is considered a health hazard due to its radioactivity. Intense radioactivity also hindered chemical studies of radon and only a few compounds are known. Sony VAIO VPCS11V9E/B Battery

Sony VAIO VPCS11X9E/B Battery

Radon is formed as part of the normal radioactive decay chain of uranium and thorium. Uranium and thorium have been around since the earth was formed and their most common isotope has a very long half-life (4.5 billion years). Uranium and thorium,radium, and thus radon, will continue to occur for millions of years at about the same concentrations as they do now.[1] Sony VAIO VPCS123FGB Battery

As the radioactive gas of radon decays, it produces new radioactive elements called radon daughters or decay products. Radon daughters are solids and stick to surfaces such as dust particles in the air. If contaminated dust is inhaled, these particles can stick to the airways of the lung and increase the risk of developing lung cancer.[2] Sony VAIO VPCS125EC Battery

Radon is responsible for the majority of the public exposure to ionizing radiation. It is often the single largest contributor to an individual's background radiation dose, and is the most variable from location to location. Radon gas from natural sources can accumulate in buildings, especially in confined areas such as attics and basements. It can also be found in some spring waters and hot springs.[3] Sony VAIO VPCS128EC Battery

Epidemiological studies have shown a clear link between breathing high concentrations of radon and incidence of lung cancer. Thus, radon is considered a significant contaminant that affects indoor air quality worldwide. According to the United States Environmental Protection Agency, radon is the second most frequent cause of lung cancer, after cigarette smoking, causing 21,000 lung cancer deaths per year in the United States. Sony VAIO VPCS129GC Battery

About 2,900 of these deaths occur among people who have never smoked. While radon is the second most frequent cause of lung cancer, it is the number one cause among non-smokers, according to EPA estimates. [4]

Physical properties

Radon is a colorless and odorless gas, and therefore not detectable by human senses alone. Sony VAIO VPCS12C7E/B Battery

At standard temperature and pressure, radon forms a monatomic gas with a density of 9.73 kg/m3,[5] about 8 times the density of the Earth's atmosphere at sea level, 1.217 kg/m3.[6] Radon is one of the heaviest gases at room temperature and is the heaviest of the noble gases. Although colorless at standard temperature and pressure, when cooled below its freezing point of 202 K (?71 °C; ?96 °F), Sony VAIO VPCS12L9E/B Battery

radon has a brilliant phosphorescence which turns from yellow to orange-red as the temperature is lowered.[7] Uponcondensation, radon glows because of the intense radiation it produces.[8]


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nony 07/01/2012 07:48

Wow...its very well written article.