AskDefine | Define technetium

Dictionary Definition

technetium n : a crystalline metallic element not found in nature; occurs as one of the fission products of uranium [syn: Tc, atomic number 43]

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see Technetium

English

Noun

  1. a metallic chemical element (symbol Tc) with an atomic number of 43.

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For etymology and more information refer to: http://elements.vanderkrogt.net/elem/tc.html (A lot of the translations were taken from that site with permission from the author)

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Extensive Definition

Technetium () is the lightest chemical element with no stable isotope. It is a synthetic element. It has atomic number 43 and is given the symbol Tc. The chemical properties of this silvery grey, crystalline transition metal are intermediate between rhenium and manganese. Its short-lived gamma-emitting nuclear isomer 99mTc (technetium-99m) is used in nuclear medicine for a wide variety of diagnostic tests. 99Tc is used as a gamma ray-free source of beta particles. The pertechnetate ion (TcO4-) has been suggested as a strong anodic corrosion inhibitor for mild steel in closed cooling systems.
Before the element was discovered, many of the properties of element 43 were predicted by Dmitri Mendeleev. Mendeleev noted a gap in his periodic table and called the element ekamanganese(Em). In 1937 its isotope 97Tc became the first predominantly artificial element to be produced, hence its name (from the Greek τεχνητός, meaning "artificial"). Most technetium produced on Earth is a by-product of fission of uranium-235 in nuclear reactors and is extracted from nuclear fuel rods. No isotope of technetium has a half-life longer than 4.2 million years (98Tc), so its detection in red giants in 1952 helped bolster the theory that stars can produce heavier elements. On Earth, technetium occurs in trace but measurable quantities as a product of spontaneous fission in uranium ore or by neutron capture in molybdenum ores.

Characteristics

Technetium is a silvery-grey radioactive metal with an appearance similar to platinum. However, it is commonly obtained as a grey powder. Its position in the periodic table is between rhenium and manganese and as predicted by the periodic law its properties are intermediate between those two elements. Technetium is unusual among the lighter elements in that it has no stable isotopes. Only technetium and promethium have no stable isotopes, but are followed by elements which do.
Technetium is therefore extremely rare on Earth. Technetium plays no natural biological role and is not normally found in the human body.
The metal form of technetium slowly tarnishes in moist air. Its oxides are TcO2 and Tc2O7. Under oxidizing conditions technetium (VII) will exist as the pertechnetate ion, TcO4-. Common oxidation states of technetium include 0, +2, +4, +5, +6 and +7. Technetium will burn in oxygen when in powder form. It dissolves in aqua regia, nitric acid, and concentrated sulfuric acid, but it is not soluble in hydrochloric acid. It has characteristic spectral lines at 363 nm, 403 nm, 410 nm, 426 nm, 430 nm, and 485 nm.
The metal form is slightly paramagnetic, meaning its magnetic dipoles align with external magnetic fields even though technetium is not normally magnetic. The crystal structure of the metal is hexagonal close-packed. Pure metallic single-crystal technetium becomes a type II superconductor at 7.46 K; irregular crystals and trace impurities raise this temperature to 11.2 K for 99.9% pure technetium powder. Below this temperature technetium has a very high magnetic penetration depth, the largest among the elements apart from niobium.
Technetium is produced in quantity by nuclear fission, and spreads more readily than many radionuclides. In spite of the importance of understanding its toxicity in animals and humans, experimental evidence is scant. It appears to have low chemical toxicity. Its radiological toxicity (per unit of mass) is a function of compound, type of radiation for the isotope in question, and the isotope half-life. Technetium-99m is particularly attractive for medical applications, as the radiation from this isotope is a gamma ray with the same wavelength as X-rays used for common medical diagnostic X-ray applications, giving it adequate penetration while causing minimal damage for a gamma photon. This, plus the extremely short half-life of this metastable nuclear isomer, followed by the relatively long half-life of the daughter isotope Tc-99 which allows it to be eliminated from the body before it decays. This leads to a relatively low dose of administered radiation in biologically dose-equivalent amounts (sieverts) for a typical Tc-99m based nuclear scan (see more on this subject below). It is well suited to the role because it emits readily detectable 140 keV gamma rays, and its half-life is 6.01 hours (meaning that about seven eighths of it decays to 99Tc in 24 hours). Klaus Schwochau's book Technetium lists 31 radiopharmaceuticals based on 99mTc for imaging and functional studies of the brain, myocardium, thyroid, lungs, liver, gallbladder, kidneys, skeleton, blood, and tumors.
When 99mTc is combined with a tin compound it binds to red blood cells and can therefore be used to map circulatory system disorders. It is commonly used to detect gastrointestinal bleeding sites. A pyrophosphate ion with 99mTc adheres to calcium deposits in damaged heart muscle, making it useful to gauge damage after a heart attack. The sulfur colloid of 99mTc is scavenged by the spleen, making it possible to image the structure of the spleen.
Radiation exposure due to diagnostic treatment involving Tc-99m can be kept low. Because 99mTc has a short half-life and high energy gamma (allowing small amounts to be easily detected), its quick decay into the far-less radioactive 99Tc results in relatively less total radiation dose to the patient, per unit of initial activity after administration. In the form administered in these medical tests (usually pertechnetate) both isotopes are quickly eliminated from the body, generally within a few days.
Like rhenium and palladium, technetium can serve as a catalyst. For certain reactions, for example the dehydrogenation of isopropyl alcohol, it is a far more effective catalyst than either rhenium or palladium. Of course, its radioactivity is a major problem in finding safe applications. Dmitri Mendeleev predicted that this missing element, as part of other predictions, would be chemically similar to manganese and gave it the name ekamanganese.
In 1877, the Russian chemist Serge Kern reported discovering the missing element in platinum ore. Kern named what he thought was the new element davyum, after the noted English chemist Sir Humphry Davy, but it was determined to be a mixture of iridium, rhodium and iron. Another candidate, lucium, followed in 1896 but it was determined to be yttrium. Then in 1908 the Japanese chemist Masataka Ogawa found evidence in the mineral thorianite which he thought indicated the presence of element 43. Ogawa named the element nipponium, after Japan (which is Nippon in Japanese). In 2004 H. K Yoshihara utilized "a record of X-ray spectrum of Ogawa's nipponium sample from thorianite [which] was contained in a photographic plate preserved by his family. The spectrum was read and indicated the absence of the element 43 and the presence of the element 75 (rhenium)."
German chemists Walter Noddack, Otto Berg and Ida Tacke (later Mrs. Noddack) reported the discovery of element 75 and element 43 in 1925 and named element 43 masurium (after Masuria in eastern Prussia, now in Poland, the region where Walter Noddack's family originated).
In 1998 John T. Armstrong of the National Institute of Standards and Technology ran "computer simulations" of the 1925 experiments and obtained results quite close to those reported by the Noddack team. He claimed that this was further supported by work published by David Curtis of the Los Alamos National Laboratory measuring the (tiny) natural occurrence of technetium. However, the Noddack's experimental results have never been reproduced, and they were unable to isolate any element 43. Debate still exists as to whether the 1925 team actually did discover element 43.

Official discovery and later history

Discovery of element 43 was finally confirmed in a 1937 experiment at the University of Palermo in Sicily conducted by Carlo Perrier and Emilio Segrè. In the summer of 1936 Segrè and his wife visited the United States, first New York at Columbia University, where he had spent time the previous summer, and then Berkeley at Ernest O. Lawrence's Radiation Laboratory. He persuaded cyclotron inventor Lawrence to let him take back some discarded cyclotron parts that had become radioactive. In early 1937 Lawrence mailed him a molybdenum foil that had been part of the deflector in the cyclotron. Segrè enlisted his experienced chemist colleague Perrier to attempt to prove through comparative chemistry that the molybdenum activity was indeed Z = 43, an element not existent in nature because of its instability against nuclear decay. With considerable difficulty they finally succeeded in isolating three distinct decay periods (90, 80, and 50 days) that eventually turned out to be two isotopes, 95Tc and 97Tc, of technetium, the name given later by Perrier and Segrè to the first man-made element. University of Palermo officials wanted them to name their discovery panormium, after the Latin name for Palermo, Panormus. The researchers instead named element 43 after the Greek word τεχνητός, meaning "artificial", since it was the first element to be artificially produced.
In 1952 astronomer Paul W. Merrill in California detected the spectral signature of technetium (in particular, light at 403.1 nm, 423.8 nm, 426.8 nm, and 429.7 nm) in light from S-type red giants. More recently, such observations provided evidence that elements were being formed by neutron capture in the s-process. There is also evidence that the Oklo natural nuclear fission reactor produced significant amounts of technetium-99, which has since decayed to ruthenium-99. Extraterrestrial technetium was found in some red giant stars (S-, M-, and N-types) that contain an absorption line in their spectrum indicating the presence of this element.

Byproduct production of Tc-99 in fission wastes

In contrast with the rare natural occurrence, bulk quantities of technetium-99 are produced each year from spent nuclear fuel rods, which contain various fission products. The fission of a gram of uranium-235 in nuclear reactors yields 27 mg of 99Tc, giving technetium a fission product yield of 6.1%. Other fissile isotopes also produce similar yields of technetium, However, only a fraction of the production is used commercially. As of 2005, technetium-99 is available to holders of an ORNL permit for US$83/g plus packing charges.
Since the yield of technetium-99 as a product of the nuclear fission of both uranium-235 and plutonium-239 is moderate, it is present in radioactive waste of fission reactors and is produced when a fission bomb is detonated. The amount of artificially produced technetium in the environment exceeds its natural occurrence to a large extent. This is due to release by atmospheric nuclear testing along with the disposal and processing of high-level radioactive waste. Due to its high fission yield and relatively high half-life, technetium-99 is one of the main components of nuclear waste. Its decay, measured in becquerels per amount of spent fuel, is dominant at about 104 to 106 years after the creation of the nuclear waste.
As a result of nuclear fuel reprocessing, technetium has been discharged into the sea in a number of locations, and some seafood contains tiny but measurable quantities. For example, lobster from west Cumbria contains small amounts of technetium. The anaerobic, spore-forming bacteria in the Clostridium genus are able to reduce Tc(VII) to Tc(IV). Clostridia bacteria play a role in reducing iron, manganese and uranium, thereby affecting these elements' solubility in soil and sediments. Their ability to reduce technetium may determine a large part of Tc's mobility in industrial wastes and other subsurface environments.
The long half-life of technetium-99 and its ability to form an anionic species makes it (along with 129I) a major concern when considering long-term disposal of high-level radioactive waste. In addition, many of the processes designed to remove fission products from medium-active process streams in reprocessing plants are designed to remove cationic species like caesium (e.g., 137Cs) and strontium (e.g., 90Sr). Hence the pertechnetate is able to escape through these treatment processes. Current disposal options favor burial in geologically stable rock. The primary danger with such a course is that the waste is likely to come into contact with water, which could leach radioactive contamination into the environment. The anionic pertechnetate and iodide are less able to absorb onto the surfaces of minerals so they are likely to be more mobile.
By comparison plutonium, uranium, and caesium are much more able to bind to soil particles. For this reason, the environmental chemistry of technetium is an active area of research. An alternative disposal method, transmutation, has been demonstrated at CERN for technetium-99. This transmutation process is one in which the technetium (99Tc as a metal target) is bombarded with neutrons to form the shortlived 100Tc (half life = 16 seconds) which decays by beta decay to ruthenium (100Ru). If recovery of usable ruthenium is a goal, an extremely pure technetium target is needed; if small traces of the minor actinides such as americium and curium are present in the target, they are likely to undergo fission and form more fission products which increase the radioactivity of the irradiated target. The formation of 106Ru (half life 374 days) from the fresh fission is likely to increase the activity of the final ruthenium metal, which will then require a longer cooling time after irradiation before the ruthenium can be used.
The actual production of technetium-99 from spent nuclear fuel is a long process. During fuel reprocessing, it appears in the waste liquid, which is highly radioactive. After sitting for several years, the radioactivity has fallen to a point where extraction of the long-lived isotopes, including technetium-99, becomes feasible. Several chemical extraction processes are used yielding technetium-99 metal of high purity. The hospital then chemically extracts the technetium from the solution by using a technetium-99m generator ("technetium cow", also occasionally called a "molybdenum cow").
The normal technetium cow is an alumina column which contains molybdenum-98; in as much as aluminium has a small neutron cross section, it is convenient for an alumina column bearing inactive 98Mo to be irradiated with neutrons to make the radioactive Mo-99 column for the technetium cow. By working in this way, there is no need for the complex chemical steps which would be required to separate molybdenum from a fission product mixture. This alternative method requires that an enriched uranium target be irradiated with neutrons to form 99Mo as a fission product, then separated.
Other technetium isotopes are not produced in significant quantities by fission; when needed, they are manufactured by neutron irradiation of parent isotopes (for example, 97Tc can be made by neutron irradiation of 96Ru).

Isotopes

Technetium is one of the two elements in the first 82 that have no stable isotopes (in fact, it is the lowest-numbered element that is exclusively radioactive); the other such element is promethium. The most stable radioisotopes are 98Tc (half-life of 4.2 Ma), 97Tc (half-life: 2.6 Ma) and 99Tc (half-life: 211.1 ka).
Twenty-two other radioisotopes have been characterized with atomic masses ranging from 87.933 u (88Tc) to 112.931 u (113Tc). Most of these have half-lives that are less than an hour; the exceptions are 93Tc (half-life: 2.75 hours), 94Tc (half-life: 4.883 hours), 95Tc (half-life: 20 hours), and 96Tc (half-life: 4.28 days).
Technetium-99 is the most common and most readily available isotope, as it is a major product of the fission of uranium-235. One gram of 99Tc produces 6.2×108 disintegrations a second (that is, 0.62 GBq/g).

Stability of technetium isotopes

Technetium and promethium are unusual light elements in that they have no stable isotopes. The reason for this is somewhat complicated.
Using the liquid drop model for atomic nuclei, one can derive a semiempirical formula for the binding energy of a nucleus. This formula predicts a "valley of beta stability" along which nuclides do not undergo beta decay. Nuclides that lie "up the walls" of the valley tend to decay by beta decay towards the center (by emitting an electron, emitting a positron, or capturing an electron).
For a fixed odd number of nucleons A, the graph of binding energies vs. atomic number (number of protons) is shaped like a parabola (U-shaped), with the most stable nuclide at the bottom. A single beta decay or electron captures then transforms one nuclide of mass A into the next or preceding one, if the product has a lower binding energy and the difference in energy is sufficient to drive the decay mode. When there is only one parabola, there can be only one stable isotope lying on that parabola. For a fixed even number of nucleons A, the graph is jagged and is better visualized as two separate parabolas for even and odd atomic numbers, because isotopes with an even number of protons and an even number of neutrons are more stable than isotopes with an odd number of neutrons and an odd number of protons.
When there are two parabolas, that is, when the number of nucleons is even, it can happen (rarely) that there is a stable nucleus with an odd number of neutrons and an odd number of protons (although there are only 4 truly stable examples as opposed to very long-lived: the light nuclei: 2H, 6Li, 10B, 14N). However, if this happens, there can be no stable isotope with an even number of neutrons and an even number of protons.
For technetium (Z=43), the valley of beta stability is centered at around 98 nucleons. However, for every number of nucleons from 95 to 102, there is already at least one stable nuclide of either molybdenum (Z=42) or ruthenium (Z=44). For the isotopes with odd numbers of nucleons, this immediately rules out a stable isotope of technetium, since there can be only one stable nuclide with a fixed odd number of nucleons. For the isotopes with an even number of nucleons, since technetium has an odd number of protons, any isotope must also have an odd number of neutrons. In such a case, the presence of a stable nuclide having the same number of nucleons and an even number of protons rules out the possibility of a stable nucleus.

References

Works cited

technetium in Afrikaans: Tegnesium
technetium in Arabic: تكنيتيوم
technetium in Azerbaijani: Texnesium
technetium in Bengali: টেকনেটিয়াম
technetium in Belarusian: Тэхнецый
technetium in Bulgarian: Технеций
technetium in Catalan: Tecneci
technetium in Czech: Technecium
technetium in Corsican: Tecneziu
technetium in Danish: Technetium
technetium in German: Technetium
technetium in Estonian: Tehneetsium
technetium in Modern Greek (1453-): Τεχνήτιο
technetium in Spanish: Tecnecio
technetium in Esperanto: Teknecio
technetium in Basque: Teknezio
technetium in Persian: تکنسیوم
technetium in French: Technétium
technetium in Friulian: Tecnezi
technetium in Manx: Çheghnaiçhum
technetium in Galician: Tecnecio
technetium in Korean: 테크네튬
technetium in Armenian: Տեխնեցիում
technetium in Hindi: टेक्निशियम
technetium in Croatian: Tehnecij
technetium in Ido: Teknecio
technetium in Indonesian: Teknesium
technetium in Icelandic: Teknetín
technetium in Italian: Tecnezio
technetium in Hebrew: טכנציום
technetium in Swahili (macrolanguage): Tekineti
technetium in Haitian: Tèknesyòm
technetium in Kurdish: Teknesyûm
technetium in Latin: Technetium
technetium in Latvian: Tehnēcijs
technetium in Luxembourgish: Technetium
technetium in Lithuanian: Technecis
technetium in Lojban: runjinme
technetium in Hungarian: Technécium
technetium in Malayalam: ടെക്നീഷ്യം
technetium in Dutch: Technetium
technetium in Japanese: テクネチウム
technetium in Norwegian: Technetium
technetium in Norwegian Nynorsk: Technetium
technetium in Occitan (post 1500): Tecnèci
technetium in Uzbek: Texnetsiy
technetium in Polish: Technet
technetium in Portuguese: Tecnécio
technetium in Romanian: Technetiu-99m
technetium in Russian: Технеций
technetium in Sicilian: Tecnezziu
technetium in Simple English: Technetium
technetium in Slovak: Technécium
technetium in Slovenian: Tehnecij
technetium in Serbian: Техницијум
technetium in Serbo-Croatian: Tehnicijum
technetium in Finnish: Teknetium
technetium in Swedish: Teknetium
technetium in Tamil: டெக்னீசியம்
technetium in Telugu: టెక్నీషియమ్
technetium in Thai: เทคนีเชียม
technetium in Vietnamese: Tecneti
technetium in Turkish: Teknesyum
technetium in Ukrainian: Технецій
technetium in Chinese: 锝
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