Technetium is a chemical element; it has symbol Tc and atomic number 43. It is the lightest element whose isotopes are all radioactive. Technetium is one of only two radioactive elements both preceded and succeeded in the periodic table by elements with stable forms, the other being promethium. 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 <sup>99</sup>Tc, 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 beta particles. Long-lived technetium isotopes produced commercially are byproducts of the 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 red giants helped to prove that stars can produce heavier elements.
History
Early assumptions
From the 1860s through 1871, early forms of the periodic table proposed by Dmitri Mendeleev contained a gap between molybdenum (element 42) and ruthenium (element 44). In 1871, Mendeleev predicted this missing element would occupy the empty place below manganese and have similar chemical properties. Mendeleev gave it the provisional name eka-manganese (from eka, the Sanskrit word for one) because it was one place down from the known element manganese.
Early misidentifications
Many early researchers, both before and after the periodic table was published, 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. This turned out not to be the case, due to technetium's radioactivity.
{| class="wikitable"
! Year
! Claimant
! Suggested name
! Actual material
|-
|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
|}
Irreproducible results
thumb|right| (Periodic system of the elements) (1904–1945, now at the [[Gdańsk University of Technology): lack of elements: polonium Po (though discovered as early as in 1898 by Maria Sklodowska-Curie), astatine At (1940, in Berkeley), francium Fr (1939, in France), neptunium Np (1940, in Berkeley) and other actinides and lanthanides. Uses old symbols for: argon Ar (here: A), technetium Tc (Ma, masurium), xenon Xe (X), radon Rn (Em, emanation).]]
German chemists Walter Noddack, Otto Berg, and Ida Tacke 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). Still, in 1933, a series of articles on the discovery of elements quoted the name masurium for element 43. Some more recent attempts have been made to rehabilitate the Noddacks' claims, but they are disproved by Paul Kuroda's study on the amount of technetium that could have been present in the ores they studied: it could not have exceeded of ore, and thus would have been undetectable by the Noddacks' methods.
Official discovery and later history
The discovery of element 43 was finally confirmed in a 1937 experiment at the University of Palermo in Sicily by Carlo Perrier and Emilio Segrè. In mid-1936, Segrè visited the United States, first Columbia University in New York and then the Lawrence Berkeley National Laboratory in California. He persuaded cyclotron inventor Ernest Lawrence to let him take back some discarded cyclotron parts that had become radioactive. Lawrence mailed him a molybdenum foil that had been part of the deflector in the cyclotron.
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 , after the Latin name for Palermo, '. In 1947, element 43 was named after the Greek word (), meaning 'artificial', since it was the first element to be artificially produced.
Segrè returned to Berkeley and met Glenn T. Seaborg. They isolated the metastable isotope technetium-99m, which is now used in some ten million medical diagnostic procedures annually.
In 1952, the astronomer Paul W. Merrill detected the spectral signature of technetium (specifically wavelengths of 403.1 nm, 423.8 nm, 426.2 nm, and 429.7 nm) in light from S-type red giants. 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 nuclear reactions. 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 pitchblende from the Belgian Congo in very small quantities (about 0.2 ng/kg), The unit cell parameters of the orthorhombic Tc metal were reported when Tc is contaminated with carbon ( = 0.2805(4), = 0.4958(8), = 0.4474(5)·nm for Tc-C with 1.38 wt% C and = 0.2815(4), = 0.4963(8), = 0.4482(5)·nm for Tc-C with 1.96 wt% C ). Pure, metallic, single-crystal technetium becomes a type-II superconductor at temperatures below .
Below this temperature, technetium has a very high magnetic penetration depth, greater than any other element except niobium.
Chemical properties
Technetium is located in group 7 of the periodic table, between rhenium and manganese. As predicted by the periodic law, its chemical properties are between those two elements. Of the two, technetium more closely resembles rhenium, particularly in its chemical inertness and tendency to form covalent bonds. This is consistent with the tendency of period 5 elements to resemble their counterparts in period 6 more than period 4 due to the lanthanide contraction. Unlike manganese, technetium does not readily form cations (ions with net positive charge). Technetium exhibits nine oxidation states from −1 to +7, with +4, +5, and +7 being the most common. 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.
Compounds
Pertechnetate and other derivatives
thumb|upright|Pertechnetate is one of the most available forms of technetium. It is structurally related to [[permanganate.]]
The most prevalent form of technetium that is easily accessible is sodium pertechnetate, Na[TcO<sub>4</sub>]. The majority of this material is produced by radioactive decay from [<sup>99</sup>MoO<sub>4</sub>]<sup>2−</sup>: and 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 centrosymmetric 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:
HTcO<sub>4</sub> is a strong acid. In concentrated sulfuric acid, [TcO<sub>4</sub>]<sup>−</sup> converts to the octahedral form TcO<sub>3</sub>(OH)(H<sub>2</sub>O)<sub>2</sub>, the conjugate base of the hypothetical triaquo complex [TcO<sub>3</sub>(H<sub>2</sub>O)<sub>3</sub>]<sup>+</sup>.
Other chalcogenide derivatives
Technetium forms a dioxide, disulfide, diselenide, and ditelluride. An ill-defined Tc<sub>2</sub>S<sub>7</sub> forms upon treating pertechnate with hydrogen sulfide. It thermally decomposes into disulfide and elemental sulfur. Similarly the dioxide can be produced by reduction of the Tc<sub>2</sub>O<sub>7</sub>.
Unlike the case for rhenium, a trioxide has not been isolated for technetium. However, TcO<sub>3</sub> has been identified in the gas phase using mass spectrometry.
Simple hydride and halide complexes
Technetium forms the complex . The potassium salt is isostructural with Potassium nonahydridorhenate|. At high pressure formation of TcH<sub>1.3</sub> from elements was also reported. These compounds are produced by combining the metal and halogen or by less direct reactions.
TcCl<sub>4</sub> is obtained by chlorination of Tc metal or Tc<sub>2</sub>O<sub>7</sub>. Upon heating, TcCl<sub>4</sub> gives the corresponding Tc(III) and Tc(II) chlorides. It is prepared by treating the chloro-acetate Tc<sub>2</sub>(O<sub>2</sub>CCH<sub>3</sub>)<sub>4</sub>Cl<sub>2</sub> with HCl. Like Re<sub>3</sub>Cl<sub>9</sub>, the structure of the α-polymorph consists of triangles with short M-M distances. β-TcCl<sub>3</sub> features octahedral Tc centers, which are organized in pairs, as seen also for molybdenum trichloride. TcBr<sub>3</sub> 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. TcI<sub>3</sub> has the same structure as the high temperature phase of TiI<sub>3</sub>, featuring chains of confacial octahedra with equal Tc—Tc contacts.
Coordination and organometallic complexes
thumb|right|[[Technetium (99mTc) sestamibi|Technetium (<sup>99m</sup>Tc) sestamibi ("Cardiolite") is widely used for imaging of the heart.]]
Technetium forms a variety of coordination complexes with organic ligands. Many have been well-investigated because of their relevance to nuclear medicine.
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. In this molecule, two technetium atoms are bound to each other; each atom is surrounded by octahedra 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 carbonyls are formed by technetium's congeners, manganese and rhenium. Interest in organotechnetium compounds has also been motivated by applications in nuclear medicine. Technetium also forms aquo-carbonyl complexes, one prominent complex being [Tc(CO)<sub>3</sub>(H<sub>2</sub>O)<sub>3</sub>]<sup>+</sup>, which are unusual compared to other metal carbonyls. and odd numbered elements have fewer stable isotopes.
The most stable radioactive isotopes 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 mass numbers 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 (<sup>98</sup>Tc) is electron capture, producing molybdenum (Z = 42).
Technetium also has numerous nuclear isomers, which are isotopes with one or more excited nucleons. Technetium-97m (<sup>97m</sup>Tc; "m" stands for metastable) is the most stable, with a half-life of 91.1 days and excitation energy 0.097 MeV, followed by technetium-95m (62.0 days, 0.039 MeV), and technetium-99m (6.01 hours, 0.143 MeV).
Technetium-99 (<sup>99</sup>Tc) is a major product of the fission of uranium-235 (<sup>235</sup>U), 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 <sup>99</sup>Tc is 0.62 GBq/g).
Some red giant stars with the spectral types S, M, and N display a spectral absorption line indicating the presence of technetium.<!--Technetium in Red Giant Stars P Merrill — Science, 1952--> These red giants are known informally as technetium stars.
Fission product
In contrast to 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 technetium-99, giving technetium a fission product yield of 6.1%.
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.
From 2000 onwards the amount has been limited by regulation to 90 TBq (about 140 kg) per year.
Discharge of technetium into the sea resulted in contamination of some seafood with minuscule quantities of this element. For example, European lobster and fish from west Cumbria contain about 1 Bq/kg of technetium.
Fission product for commercial use
The metastable isotope technetium-99m is continuously produced as a fission product from the fission of uranium or plutonium in nuclear reactors:
<chem display="block"> ^{238}_{92}U ->[\ce{sf}] ^{137}_{53}I + ^{99}_{39}Y + 2^{1}_{0}n</chem>
<chem display="block"> ^{99}_{39}Y ->[\beta^-][1.47\,\ce{s}] ^{99}_{40}Zr ->[\beta^-][2.1\,\ce{s}] ^{99}_{41}Nb ->[\beta^-][15.0\,\ce{s}] ^{99}_{42}Mo ->[\beta^-][65.94\,\ce{h}] ^{99}_{43}Tc ->[\beta^-][211,100\,\ce{y}] ^{99}_{44}Ru</chem>
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 actinides 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. Molybdenum-99 in the form of molybdate is adsorbed onto acid alumina () in a shielded column chromatograph inside a technetium-99m generator ("technetium cow", also occasionally called a "molybdenum cow"). Molybdenum-99 has a half-life of 67 hours, so short-lived technetium-99m (half-life: 6 hours), which results from its decay, is being constantly produced.
thumb|upright|The first [[technetium-99m generator, unshielded, 1958. A Tc-99m pertechnetate solution is being eluted from Mo-99 molybdate bound to a chromatographic substrate]]
Almost two-thirds of the world's supply comes from two reactors; the National Research Universal Reactor at Chalk River Laboratories in Ontario, Canada, and the High Flux Reactor at Nuclear Research and Consultancy Group in Petten, Netherlands. All major reactors that produce technetium-99m were built in the 1960s and are close to the end of life. The two new Canadian Multipurpose Applied Physics Lattice Experiment reactors planned and built to produce 200% of the demand of technetium-99m relieved all other producers from building their own reactors. With the cancellation of the already tested reactors in 2008, the future supply of technetium-99m became problematic.
Waste disposal
The long half-life of technetium-99 and its potential to form anionic species creates a major concern for long-term disposal of radioactive waste. Many of the processes designed to remove fission products in reprocessing plants aim at cationic species such as caesium (e.g., caesium-137) and strontium (e.g., strontium-90). Hence the pertechnetate escapes through those processes. Current disposal options favor burial in continental, geologically stable rock. The primary danger with such practice is the likelihood that the waste will contact water, which could leach radioactive contamination into the environment. The anionic pertechnetate and iodide tend not to adsorb into the surfaces of minerals, and are likely to be washed away. By comparison plutonium, uranium, and caesium tend to bind to soil particles. Technetium could be immobilized by some environments, such as microbial activity in lake bottom sediments, and the environmental chemistry of technetium is an area of active research.
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 neutrons 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 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 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<!--isotopic and chemical?--> purity.
Neutron activation
Molybdenum-99, which decays to form technetium-99m, can be formed by the neutron activation of molybdenum-98. When needed, other technetium isotopes are not produced in significant quantities by fission, but are manufactured by neutron irradiation of parent isotopes (for example, technetium-97 can be made by neutron irradiation of ruthenium-96).
Particle accelerators
The feasibility of technetium-99m production with the 22-MeV-proton bombardment of a molybdenum-100 target in medical cyclotrons following the reaction <sup>100</sup>Mo(p,2n)<sup>99m</sup>Tc was demonstrated in 1971. The recent shortages of medical technetium-99m reignited the interest in its production by proton bombardment of isotopically enriched (>99.5%) molybdenum-100 targets. Other techniques are being investigated for obtaining molybdenum-99 from molybdenum-100 via (n,2n) or (γ,n) reactions in particle accelerators.
Applications
Nuclear medicine and biology
thumb|upright|Technetium [[Nuclear medicine|scintigraphy of a neck of Graves' disease patient|alt=Upper image: two drop-like features merged at their bottoms; they have a yellow centre and a red rim on a black background. Caption: Graves' Disease Tc-Uptake 16%. Lower image: red dots on black background. Caption: 250 Gy (30mCi) + Prednison.]]
Technetium-99m ("m" indicates that this is a metastable nuclear isomer) is used in radioactive isotope medical tests. For example, technetium-99m is a radioactive tracer that medical imaging equipment tracks in the human body.
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. In the body, technetium is quickly converted to the stable ion, which is highly water-soluble and quickly excreted. The radiological toxicity of technetium (per unit of mass) is a function of compound, type of radiation for the isotope in question, and the isotope's half-life.
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. <!-- NEEDS CITE Soft X-rays are emitted when the beta particles are stopped, but as long as the body is kept more than away these should pose no problem. /NEEDS CITE --> 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.
Notes
References
S. Garg and B. Maheshwari, et al., Atomic Data and Nuclear Data Tables 150, 101546 (2023) https://doi.org/10.1016/j.adt.2022.101546