Technetium is a chemical element; it has symbol Tc and atomic number 43. It is the lightest element whose isotopes are all radioactive. Technetium and promethium are the only radioactive elements whose neighbours in the sense of atomic number are both stable. All available technetium is produced as a synthetic element. Naturally occurring technetium is a spontaneous fission product in uranium ore and thorium ore (the most common source), or the product of neutron capture in molybdenum ores. This silvery gray, crystalline transition metal lies between manganese and rhenium in group 7 of the periodic table, and its chemical properties are intermediate between those of both adjacent elements. The most common naturally occurring isotope is 99Tc, in traces only.
Many of technetium's properties had been predicted by Dmitri Mendeleev before it was discovered; Mendeleev noted a gap in his periodic table and gave the undiscovered element the provisional name ekamanganese ( Em). In 1937, technetium became the first predominantly artificial element to be produced, hence its name (from the Greek , 'artificial', +
One short-lived gamma ray–emitting nuclear isomer, technetium-99m, is used in nuclear medicine for a wide variety of tests, such as bone cancer diagnoses. The ground state of the nuclide technetium-99 is used as a gamma ray–free source of . Long-lived technetium isotopes produced commercially are byproducts of the nuclear fission of uranium-235 in nuclear reactors and are extracted from nuclear fuel rods. Because even the longest-lived isotope of technetium has a relatively short half-life (4.21 million years), the 1952 detection of technetium in helped to prove that stars can nuclear fusion.
1828 | Gottfried Osann | Polinium | Iridium |
1845 | Heinrich Rose | Pelopium | Niobium–tantalum alloy |
1847 | R. Hermann | Ilmenium | Niobium–tantalum alloy |
1877 | Serge Kern | Davyum | Iridium–rhodium–iron alloy |
1896 | Prosper Barrière | Lucium | Yttrium |
1908 | Masataka Ogawa | Nipponium | Rhenium, which was the unknown dvi-manganese |
Segrè enlisted his colleague Perrier to attempt to prove, through comparative chemistry, that the molybdenum activity was indeed from an element with the atomic number 43, which they did. University of Palermo officials wanted them to name their discovery panormium, after the Latin name for Palermo, Panormus. In 1947, element 43 was named after the Greek language word (τεχνητός), meaning 'artificial', since it was the first element to be artificially produced.
In 1952, the astronomer Paul W. Merrill detected the spectral signature of technetium (specifically of 403.1 Nanometre, 423.8 nm, 426.2 nm, and 429.7 nm) in light from S-type . The stars were near the end of their lives but were rich in the short-lived element, which indicated that it was being produced in the stars by . That evidence bolstered the hypothesis that heavier elements are the product of nucleosynthesis in stars. More recently, such observations provided evidence that elements are formed by neutron capture in the s-process.
Since that discovery, there have been many searches in terrestrial materials for natural sources of technetium. In 1962, technetium-99 was isolated and identified in uraninite from the Belgian Congo in very small quantities (about 0.2 ng/kg), where it originates as a spontaneous fission product of uranium-238. The natural nuclear fission reactor in Oklo contains evidence that significant amounts of technetium-99 were produced and have since decayed into ruthenium-99.
Metallic technetium slowly in moist air and, in powder form, burns in oxygen. When reacting with hydrogen at high pressure, it forms the non-stoichiometric hydride TcH and while reacting with carbon it forms TcC, with cell parameter 0.398 nm.
Technetium can catalyse the destruction of hydrazine by nitric acid, and this property is due to its multiplicity of valencies. This caused a problem in the separation of plutonium from uranium in nuclear fuel processing, where hydrazine is used as a protective reductant to keep plutonium in the trivalent rather than the more stable tetravalent state. The problem was exacerbated by the mutually enhanced solvent extraction of technetium and zirconium at the previous stage, and required a process modification.
Pertechnetate () is only weakly hydrated in aqueous solutions, and it behaves analogously to perchlorate anion, both of which are tetrahedral. Unlike permanganate (), it is only a weak oxidizing agent.
Related to pertechnetate is technetium heptoxide. This pale-yellow, volatile solid is produced by oxidation of Tc metal and related precursors:
It is a molecular metal oxide, analogous to manganese heptoxide. It adopts a Centrosymmetry structure with two types of Tc−O bonds with 167 and 184 pm bond lengths.
Technetium heptoxide hydrolyzes to pertechnetate and pertechnetic acid, depending on the pH:
HTcO4 is a strong acid. In concentrated sulfuric acid, TcO4− converts to the octahedral form TcO3(OH)(H2O)2, the conjugate base of the hypothetical triaquo complex TcO3(H2O)3+.
Unlike the case for rhenium, a trioxide has not been isolated for technetium. However, TcO3 has been identified in the gas phase using mass spectrometry.
The following binary (containing only two elements) technetium halides are known: TcF6, TcF5, TcCl4, TcBr4, TcBr3, α-TcCl3, β-TcCl3, TcI3, α-TcCl2, and β-TcCl2. The range from Tc(VI) to Tc(II). Technetium halides exhibit different structure types, such as molecular octahedral complexes, extended chains, layered sheets, and metal clusters arranged in a three-dimensional network. These compounds are produced by combining the metal and halogen or by less direct reactions.
TcCl4 is obtained by chlorination of Tc metal or Tc2O7. Upon heating, TcCl4 gives the corresponding Tc(III) and Tc(II) chlorides.
The structure of TcCl4 is composed of infinite zigzag chains of edge-sharing TcCl6 octahedra. It is isomorphous to transition metal tetrachlorides of zirconium, hafnium, and platinum.
Two polymorphs of technetium trichloride exist, α- and β-TcCl3. The α polymorph is also denoted as Tc3Cl9. It adopts a confacial bioctahedral structure. It is prepared by treating the chloro-acetate Tc2(O2CCH3)4Cl2 with HCl. Like Re3Cl9, the structure of the α-polymorph consists of triangles with short M-M distances. β-TcCl3 features octahedral Tc centers, which are organized in pairs, as seen also for molybdenum trichloride. TcBr3 does not adopt the structure of either trichloride phase. Instead it has the structure of molybdenum tribromide, consisting of chains of confacial octahedra with alternating short and long Tc—Tc contacts. TcI3 has the same structure as the high temperature phase of TiI3, featuring chains of confacial octahedra with equal Tc—Tc contacts.
Several anionic technetium halides are known. The binary tetrahalides can be converted to the hexahalides TcX62− (X = F, Cl, Br, I), which adopt octahedral molecular geometry. More reduced halides form anionic clusters with Tc–Tc bonds. The situation is similar for the related elements of Mo, W, Re. These clusters have the nuclearity Tc4, Tc6, Tc8, and Tc13. The more stable Tc6 and Tc8 clusters have prism shapes where vertical pairs of Tc atoms are connected by triple bonds and the planar atoms by single bonds. Every technetium atom makes six bonds, and the remaining valence electrons can be saturated by one axial and two bridging ligand halogen atoms such as chlorine or bromine.
Technetium forms a variety of compounds with Tc–C bonds, i.e. organotechnetium complexes. Prominent members of this class are complexes with CO, arene, and cyclopentadienyl ligands. The binary carbonyl Tc2(CO)10 is a white volatile solid. In this molecule, two technetium atoms are bound to each other; each atom is surrounded by octahedron of five carbonyl ligands. The bond length between technetium atoms, 303 pm, is significantly larger than the distance between two atoms in metallic technetium (272 pm). Similar are formed by technetium's congeners, manganese and rhenium. Interest in organotechnetium compounds has also been motivated by applications in nuclear medicine.
The most stable Radionuclide are technetium-97 with a half-life of million years and technetium-98 with million years; current measurements of their half-lives give overlapping confidence intervals corresponding to one standard deviation and therefore do not allow a definite assignment of technetium's most stable isotope. The next most stable isotope is technetium-99, which has a half-life of 211,100 years. Thirty-four other radioisotopes have been characterized with ranging from 86 to 122. Most of these have half-lives that are less than an hour, the exceptions being technetium-93 (2.75 hours), technetium-94 (4.88 hours), technetium-95 (19.26 hours), and technetium-96 (4.28 days).
The primary decay mode for isotopes lighter than technetium-98 (98Tc) is electron capture, producing molybdenum ( Z = 42). For technetium-98 and heavier isotopes, the primary mode is Beta decay (the emission of an electron or positron), producing ruthenium ( Z = 44), with the exception that technetium-100 can decay both by beta emission and electron capture.
Technetium also has numerous , which are isotopes with one or more Excited state nucleons. Technetium-97m (97mTc; "m" stands for metastability) is the most stable, with a half-life of 91.1 days and excited state 0.097 MeV, followed by technetium-95m (62.0 days, 0.039 MeV), and technetium-99m (6.01 hours, 0.143 MeV).
Technetium-99 (99Tc) is a major product of the fission of uranium-235 (235U), making it the most common and most readily available isotope of technetium, and the only one detected in nature. One gram of technetium-99 produces per second (in other words, the specific activity of 99Tc is 0.62 GBecquerel/g).
Technetium-99 is produced by the nuclear fission of both uranium-235 and plutonium-239. It is therefore present in radioactive waste and in the nuclear fallout of nuclear weapon explosions. Its decay, measured in per amount of spent fuel, is the dominant contributor to nuclear waste radioactivity after about after the creation of the nuclear waste. From 1945 to 1994, an estimated 160 TBecquerel (about 250 kg) of technetium-99 was released into the environment during atmospheric .
The amount of technetium-99 from nuclear reactors released into the environment up to 1986 is on the order of 1000 TBq (about 1600 kg), primarily by nuclear fuel reprocessing; most of this was discharged into the sea. Reprocessing methods have reduced emissions since then, 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 to 1999 into the Irish Sea.
Because used fuel is allowed to stand for several years before reprocessing, all molybdenum-99 and technetium-99m is decayed by the time that the fission products are separated from the major in conventional nuclear reprocessing. The liquid left after plutonium–uranium extraction (PUREX) contains a high concentration of technetium as but almost all of this is technetium-99, not technetium-99m.
The vast majority of the technetium-99m used in medical work is produced by irradiating dedicated highly enriched uranium targets in a reactor, extracting molybdenum-99 from the targets in reprocessing facilities, and recovering at the diagnostic center the technetium-99m produced upon decay of molybdenum-99.
An alternative disposal method, transmutation, has been demonstrated at CERN for technetium-99. In this process, the technetium (technetium-99 as a metal target) is bombarded with to form the short-lived technetium-100 (half-life = 16 seconds) which decays by beta decay to stable ruthenium-100. If recovery of usable ruthenium is a goal, an extremely pure technetium target is needed; if small traces of the such as americium and curium are present in the target, they are likely to undergo fission and form more which increase the radioactivity of the irradiated target. The formation of ruthenium-106 (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 separation of technetium-99 from spent nuclear fuel is a long process. During fuel reprocessing, it comes out as a component of the highly radioactive waste liquid. After sitting for several years, the radioactivity reduces to a level where extraction of the long-lived isotopes, including technetium-99, becomes feasible. A series of chemical processes yields technetium-99 metal of high purity.
The longer-lived isotope, technetium-95m 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. In processes such as the dehydrogenation of isopropyl alcohol, it is a far more effective catalyst than either rhenium or palladium. However, its radioactivity is a major problem in safe catalytic applications.
When steel is immersed in water, adding a small concentration (55 ppm) of potassium pertechnetate(VII) to the water protects the steel from corrosion, even if the temperature is raised to . For this reason, pertechnetate has been used as an anodic corrosion inhibitor for steel, although technetium's radioactivity poses problems that limit this application to self-contained systems. While (for example) can also inhibit corrosion, it requires a concentration ten times as high. In one experiment, a specimen of carbon steel 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 (passivation). 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. 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.
As noted, the radioactive nature of technetium (3 MBq/L 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.
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. 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, and a glove box is not needed.
Being close to noble metals, technetium is not very susceptible to corrosion, and during biofouling, its ability to self-cleanse has been recorded due to its radiotoxic effect on biota.
Characteristics
Physical properties
Chemical properties
Compounds
Pertechnetate and other derivatives
Other chalcogenide derivatives
Simple hydride and halide complexes
Coordination and organometallic complexes
Isotopes
Occurrence and production
Fission product
Fission product for commercial use
Waste disposal
Neutron activation
Particle accelerators
Applications
Nuclear medicine and biology
Industrial and chemical
Precautions and biological effect
Notes
Bibliography
Further reading
External links
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