2007 Schools Wikipedia Selection. Related subjects: Chemical elements
|Name, Symbol, Number||technetium, Tc, 43|
|Chemical series||transition metals|
|Group, Period, Block||7, 5, d|
|Appearance||silvery gray metal|
|Atomic mass||(0) g/mol|
|Electron configuration||[Kr] 4d5 5s2|
|Electrons per shell||2, 8, 18, 13, 2|
|Density (near r.t.)||11 g·cm−3|
|Melting point||2430 K
(2157 ° C, 3915 ° F)
|Boiling point||4538 K
(4265 ° C, 7709 ° F)
|Heat of fusion||33.29 kJ·mol−1|
|Heat of vaporization||585.2 kJ·mol−1|
|Heat capacity||(25 °C) 24.27 J·mol−1·K−1|
(strongly acidic oxide)
|Electronegativity||1.9 (Pauling scale)|
|Electron affinity||-53 kJ/mol|
|Ionization energies||1st: 702 kJ/mol|
|2nd: 1470 kJ/mol|
|3rd: 2850 kJ/mol|
|Atomic radius||135 pm|
|Atomic radius (calc.)||183 pm|
|Covalent radius||156 pm|
|Magnetic ordering||no data|
|Thermal conductivity||(300 K) 50.6 W·m−1·K−1|
|CAS registry number||7440-26-8|
Technetium ( IPA: /tɛkˈniʃɪəm/ or /tɛkˈniːʃɪəm/) is a chemical element that has the symbol Tc and the atomic number 43. The chemical properties of this silvery grey, radioactive, crystalline transition metal are intermediate between rhenium and manganese. Its short-lived isotope 99 mTc is used in nuclear medicine for a wide variety of diagnostic tests. 99Tc is used as a gamma ray-free source of beta particles, and its pertechnetate ion (TcO4-) could find use as an anodic corrosion inhibitor for steel.
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. In 1937 its isotope 97Tc became the first element to be artificially produced, hence its name (from the Greek τεχνητος, 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 naturally only in uranium ores as a product of spontaneous fission; the quantities are minute but have been measured.
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. This element is unusual among the lighter elements in that it has no stable isotopes and 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. When in powder form technetium will burn in oxygen. 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, and even lower radiological toxicity.
When one is working in a laboratory context, all isotopes of technetium must be handled carefully. The most common isotope, technetium-99, is a weak beta emitter; such radiation is stopped by the walls of laboratory glassware. Soft X-rays are emitted when the beta particles are stopped, but as long as the body is kept more than 30 cm away these should pose no problem. The primary hazard when working with technetium is inhalation of dust; such radioactive contamination in the lungs can pose a significant cancer risk. For most work, careful handling in a fume hood is sufficient; a glove box is not needed.
99mTc ("m" indicates that this is a metastable nuclear isomer) is used in radioactive isotope medical tests, for example as a radioactive tracer that medical equipment can detect in the body. 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 fifteen sixteenths 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.
Immunoscintigraphy incorporates 99mTc into a monoclonal antibody, an immune system protein capable of binding to cancer cells. A few hours after injection, medical equipment is used to detect the gamma rays emitted by the 99mTc; higher concentrations indicate where the tumor is. This technique is particularly useful for detecting hard-to-find cancers, such as those affecting the intestine. These modified antibodies are sold by the German company Hoechst under the name " Scintium".
When 99mTc is combined with a tin compound it binds to red blood cells and can therefore be used to map circulatory system disorders. 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. While 99mTc is quite radioactive (allowing small amounts to be easily detected) it has a short half-life, after which it decays into the less radioactive 99Tc. In the form administered in these medical tests (usually pertechnetate) both isotopes are quickly eliminated from the body, generally within a few days.
Technetium-99 decays almost entirely by beta decay, emitting beta particles with very consistent low energies and no accompanying gamma rays. Moreover, its very long half-life means that this emission decreases very slowly with time. It can also be extracted to a high chemical and isotopic purity from radioactive waste. For these reasons, it is a NIST standard beta emitter, used for equipment calibration.
95mTc, with a half-life of 61 days, is used as a radioactive tracer to study the movement of technetium in the environment and in plant and animal systems.
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.
Under certain circumstances, a small concentration (5×10−5 mol/ L) of the pertechnetate ion in water can protect iron and carbon steels from corrosion. For this reason, pertechnetate could find use as an anodic corrosion inhibitor for steel, although technetium's radioactivity poses problems. While (for example) CrO42− can also inhibit corrosion, it requires a concentration ten times as high. In one experiment, a test specimen was kept in an aqueous solution of pertechnetate for 20 years and was still uncorroded. The mechanism by which pertechnetate prevents corrosion is not well-understood, but seems to involve the reversible formation of a thin surface layer. One theory holds that the pertechnetate reacts with the steel surface to form a layer of technetium dioxide which prevents further corrosion; the same effect explains how iron powder can be used to remove pertechnetate from water. ( Activated carbon can also be used for the same effect.) The effect disappears rapidly if the concentration of pertechnetate falls below the minimum concentration or if too high a concentration of other ions is added. The radioactive nature of technetium (3 M Bq per liter at the concentrations required) makes this corrosion protection impractical in almost all situations. Nevertheless, corrosion protection by pertechnetate ions was proposed (but never adopted) for use in boiling water reactors.
Technetium-99 has also been proposed for use in optolectric nuclear batteries. 99Tc's beta decay electrons would stimulate an excimer mixture, and the light would power a photocell. The battery would consist of an excimer mixture of argon/xenon in a pressure vessel with an internal mirrored surface, finely divided 99Tc, and an intermittent ultrasonic stirrer, illuminating a photocell with a bandgap tuned for the excimer. If the pressure-vessel is carbon fibre/ epoxy, the weight to power ratio is said to be comparable to an air-breathing engine with fuel tanks.
For a number of years there was a gap in the periodic table between molybdenum (element 42) and ruthenium (element 44). Many early researchers were eager to be the first to discover and name the missing element; its location in the table suggested that it should be easier to find than other undiscovered elements. It was first thought to have been found in platinum ores in 1828. It was given the name polinium but it turned out to be impure iridium. Then in 1846 the element ilmenium was claimed to have been discovered but was determined to be impure niobium. This mistake was repeated in 1847 with the "discovery" of pelopium. 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 for what he thought indicated the presence of element 43. Ogawa named the element nipponium, after Japan (which is Nippon in Japanese). Later analysis indicated the presence of rhenium (element 75), not element 43.
Disputed 1925 discovery
German chemists Walter Noddack, Otto Berg and Ida Tacke (later Mrs. Noddack) reported the discovery of element 43 in 1925 and named it masurium (after Masuria in eastern Prussia). The group bombarded columbite with a beam of electrons and deduced element 43 was present by examining X-ray diffraction spectrograms. The wavelength of the X-rays produced is related to the atomic number by a formula derived by Henry Moseley in 1913. The team claimed to detect a faint X-ray signal at a wavelength produced by element 43. Contemporary experimenters could not replicate the discovery, and in fact it was dismissed as an error for many years.
It was not until 1998 that this dismissal began to be questioned. John T. Armstrong of the National Institute of Standards and Technology ran computer simulations of the experiments and obtained results very close to those reported by the 1925 team; the claim was further supported by work published by David Curtis of the Los Alamos National Laboratory measuring the (tiny) natural occurrence of technetium. Debate still exists as to whether the 1925 team actually did discover element 43.
Official discovery and later history
Discovery of element 43 has traditionally been assigned to a 1937 experiment in Sicily conducted by Carlo Perrier and Emilio Segrè. The University of Palermo researchers found the technetium isotope 97Tc in a sample of molybdenum given to Segrè by Ernest Lawrence the year before (Segrè visited Berkeley in the summer of 1936). The sample had previously been bombarded by deuterium nuclei in the University of California, Berkeley cyclotron for several months. University of Palermo officials tried unsuccessfully to force them to name their discovery panormium, after the Latin name for Palermo, Panormus. The researchers instead named element 43 after the Greek word technètos, 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. These massive stars near the end of their lives were rich in this short-lived element, meaning nuclear reactions within the stars must be producing it. This evidence was used to bolster the then unproven theory that stars are where heavier elements are produced. More recently, such observations provided evidence that elements were being formed by neutron capture in the s-process.
Since its discovery, there have been many searches in terrestrial materials for natural sources. In 1962, technetium-99 was isolated and identified in pitchblende from the Belgian Congo in very small quantities (about 0.2 ng/kg); there it originates as a spontaneous fission product of uranium-238. This discovery was made by B.T. Kenna and P.K. Kuroda. There is also evidence that the Oklo natural nuclear fission reactor produced significant amounts of technetium-99, which has since decayed to ruthenium-99.
Occurrence and production
Since technetium is unstable, only minute traces occur naturally in the Earth's crust as a spontaneous fission product of uranium. In 1999 David Curtis (see above) estimated that a kilogram of uranium contains 1 nanogram (1×10−9 g) of technetium. 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.
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 the rare isotope uranium-235 in nuclear reactors yields 27 mg of 99Tc, giving technetium a fission yield of 6.1%. Other fissionable isotopes also produce similar yields of technetium.
It is estimated that up to 1994, about 49,000 T Bq (78 metric tons) of technetium was produced in nuclear reactors, which is by far the dominant source of terrestrial 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.
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 meta stable (a state where the nucleus is in an excited state) isotope 99mTc is produced as a fission product from the fission of uranium or plutonium in nuclear reactors. Due to the fact that used fuel is allowed to stand for several years before reprocessing, all 99Mo and 99mTc will have decayed by the time that the fission products are separated from the major actinides in conventional nuclear reprocessing. The PUREX raffinate will contain a high concentration of technetium as TcO4- but almost all of this will be 99Tc. The vast majority of the 99mTc used in medical work is formed from 99Mo which is formed by the neutron activation of 98Mo. 99Mo has a half-life of 67 hours, so short-lived 99mTc (half-life: 6 hours), which results from its decay, is being constantly produced. The hospital then chemically extracts the technetium from the solution by using a technetium-99m generator ("technetium cow").
The normal technetium cow is an alumina column which contains molybdenum, as aluminium has a small neutron cross sectional it would be likely that an alumina column bearing inactive 98Mo could be irradated with neutrons to make the radioactive 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 the fission product mixture. As an alternative method, an enriched uranium target can be irradated with neutrons to form 99Mo as a fission product.
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).
Part of radioactive waste
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.
An estimated 160 T Bq (about 250 kg) of technetium-99 was released into the environment up to 1994 by atmospheric nuclear tests. The amount of technetium-99 from nuclear reactors released into the environment up to 1986 is estimated to be on the order of 1000 TBq (about 1600 kg), primarily by nuclear fuel reprocessing; most of this was discharged into the sea. In recent years, reprocessing methods have improved to reduce emissions, but as of 2005 the primary release of technetium-99 into the environment is by the Sellafield plant, which released an estimated 550 TBq (about 900 kg) from 1995-1999 into the Irish Sea. From 2000 onwards the amount has been limited by regulation to 90 TBq (about 140 kg) per year.
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 cesium (e.g., 137Cs) and strontium (e.g., 90Sr). Hence the pertechinate is able to escape through these treatment processes. Current disposal options favour 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 pertechinate and iodide are less able to absorb onto the surfaces of minerals so they are likely to be more mobile. For comparison plutonium, uranium, and cesium 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). One disadvantage of this process is the need for a very pure technetium target, while small traces of other fission products are likely to slightly increase the activity of the irradated target if small traces of the minor actinides (such as americium and curium) are present in the target then they are likely to undergo fission to form fission products. In this way a small activity and amount of minor actinides leads to a very high level of radioactivity in the irradated 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 irradation before the ruthenium can be used.
Technetium is one of the two elements in the first 82 that have no stable isotopes. 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 also has numerous meta states. 97mTc is the most stable, with a half-life of 90.1 days (0.097 MeV). This is followed by 95mTc (half life: 61 days, 0.038 MeV), and 99mTc (half-life: 6.01 hours, 0.143 MeV). 99mTc only emits gamma rays, subsequently decaying to 99Tc.
For isotopes lighter than the most stable isotope, 98Tc, the primary decay mode is electron capture, giving molybdenum. For the heavier isotopes, the primary mode is beta emission, giving ruthenium, with the exception that 100Tc can decay both by beta emission and electron capture.
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 G Bq/g).
Stability of technetium isotopes
Technetium and promethium are remarkable among the 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 centre (by emitting an electron, emitting a positron, or capturing an electron). For a fixed number of nucleons A, the binding energies lie on one or more parabolas, with the most stable nuclide at the bottom. One can have more than one parabola 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. A single beta decay then transforms one into the other. When there is only one parabola, there can be only one stable isotope lying on that parabola. 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 this happens only in four instances). 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.