Many of technetium's properties had been predicted byDmitri Mendeleev before it was discovered; Mendeleev noted a gap in his periodic table and gave the undiscovered element the provisional nameekamanganese (Em). In 1937, technetium became the first predominantly artificial element to be produced, hence its name (from the Greektechnetos, 'artificial', +-ium).
From the 1860s through 1871, early forms of the periodic table proposed byDmitri Mendeleev contained a gap betweenmolybdenum (element 42) andruthenium (element 44). In 1871, Mendeleev predicted this missing element would occupy the empty place belowmanganese and have similar chemical properties. Mendeleev gave it the provisional nameeka-manganese (fromeka, theSanskrit word forone) because it was one place down from the known element manganese.[6]
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.
Periodisches System der Elemente (Periodic system of the elements) (1904–1945, now at theGdańsk University of Technology): lack of elements:polonium84Po (though discovered as early as in 1898 byMaria Sklodowska-Curie),astatine85At (1940, in Berkeley),francium87Fr (1939, in France), neptunium93Np (1940, in Berkeley) and otheractinides andlanthanides. Uses old symbols for:argon18Ar (here: A),technetium43Tc (Ma, masurium),xenon54Xe (X),radon86Rn (Em, emanation).
German chemistsWalter Noddack,Otto Berg, andIda Tacke reported the discovery of element 75 and element 43 in 1925, and named element 43masurium (afterMasuria in easternPrussia, now inPoland, the region where Walter Noddack's family originated).[11] This name caused significant resentment in the scientific community, because it was interpreted as referring to aseries ofvictories of the German army over the Russian army in the Masuria region during World War I; as the Noddacks remained in their academic positions while the Nazis were in power, suspicions and hostility against their claim for discovering element 43 continued.[12] The group bombardedcolumbite with a beam ofelectrons and deduced element 43 was present by examiningX-ray emissionspectrograms.[13] Thewavelength of the X-rays produced is related to the atomic number by aformula derived byHenry Moseley in 1913. The team claimed to detect a faint X-ray signal at a wavelength produced by element 43. Later experimenters could not replicate the discovery, and it was dismissed as an error.[14][15] Still, in 1933, a series of articles on the discovery of elements quoted the namemasurium for element 43.[16] Some more recent attempts have been made to rehabilitate the Noddacks' claims, but they are disproved byPaul Kuroda's study on the amount of technetium that could have been present in the ores they studied: it could not have exceeded3 × 10−11 μg/kg of ore, and thus would have been undetectable by the Noddacks' methods.[12][17]
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. In 1937, they succeeded in isolating theisotopestechnetium-95m andtechnetium-97.[20][21][disputed –discuss]University of Palermo officials wanted them to name their discoverypanormium, after the Latin name forPalermo,Panormus. In 1947,[20] element 43 was named after theGreek wordtechnetos (τεχνητός), meaning 'artificial', since it was the first element to be artificially produced.[7][11]Segrè returned to Berkeley and metGlenn T. Seaborg. They isolated themetastable isotopetechnetium-99m, which is now used in some ten million medical diagnostic procedures annually.[22]
In 1952, the astronomerPaul W. Merrill detected thespectral signature of technetium (specificallywavelengths of 403.1 nm, 423.8 nm, 426.2 nm, and 429.7 nm) in light fromS-typered giants.[23] 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 bynuclear reactions. That evidence bolstered the hypothesis that heavier elements are the product ofnucleosynthesis in stars.[21] More recently, such observations provided evidence that elements are formed byneutron capture in thes-process.[24]
Since that discovery, there have been many searches in terrestrial materials for natural sources of technetium. In 1962, technetium-99 was isolated and identified inpitchblende from theBelgian Congo in very small quantities (about 0.2 ng/kg),[24] where it originates as aspontaneous fission product ofuranium-238. Thenatural nuclear fission reactor inOklo contains evidence that significant amounts of technetium-99 were produced and have since decayed intoruthenium-99.[24]
Technetium is a silvery-gray radioactivemetal with an appearance similar toplatinum, commonly obtained as a gray powder.[25] Thecrystal structure of the bulk pure metal ishexagonalclose-packed. Atomic technetium has characteristicemission lines atwavelengths of 363.3 nm, 403.1 nm, 426.2 nm, 429.7 nm, and 485.3 nm.[26] The unit cell parameters of the orthorhombic Tc metal were reported when Tc is contaminated with carbon (a = 0.2805(4),b = 0.4958(8),c = 0.4474(5)·nm for Tc-C with 1.38 wt% C anda = 0.2815(4),b = 0.4963(8),c = 0.4482(5)·nm for Tc-C with 1.96 wt% C ).[27] The metal form is slightlyparamagnetic, meaning itsmagnetic dipoles align with externalmagnetic fields, but will assume random orientations once the field is removed.[28] Pure, metallic, single-crystal technetium becomes atype-II superconductor at temperatures below 7.46 K (−265.69 °C; −446.24 °F).[29][b]Below this temperature, technetium has a very highmagnetic penetration depth, greater than any other element exceptniobium.[30]
Technetium is located in thegroup 7 of the periodic table, betweenrhenium andmanganese. As predicted by theperiodic 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 formcovalent bonds.[31] This is consistent with the tendency ofperiod 5 elements to resemble their counterparts in period 6 more than period 4 due to thelanthanide contraction. Unlike manganese, technetium does not readily formcations (ions with net positive charge). Technetium exhibits nineoxidation states from −1 to +7, with +4, +5, and +7 being the most common.[32] Technetium dissolves inaqua regia,nitric acid, and concentratedsulfuric acid, butnot inhydrochloric acid of any concentration.[25]
Metallic technetium slowlytarnishes in moist air[32] and, in powder form, burns inoxygen. When reacting withhydrogen at high pressure, it forms the hydride TcH1.3[33] and while reacting withcarbon it forms Tc6C,[27] with cell parameter 0.398 nm.
Technetium can catalyse the destruction ofhydrazine bynitric acid, and this property is due to its multiplicity of valencies.[34] This caused a problem in the separation of plutonium from uranium innuclear 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,[35] and required a process modification.
Pertechnetate is one of the most available forms of technetium. It is structurally related topermanganate.
The most prevalent form of technetium that is easily accessible issodium pertechnetate, Na[TcO4]. The majority of this material is produced by radioactive decay from [99MoO4]2−:[36][37]
Related to pertechnetate istechnetium heptoxide. This pale-yellow, volatile solid is produced by oxidation of Tc metal and related precursors:
4 Tc + 7 O2 → 2 Tc2O7
It is a molecular metal oxide, analogous tomanganese heptoxide. It adopts acentrosymmetric structure with two types of Tc−O bonds with 167 and 184 pm bond lengths.[39]
HTcO4 is a strong acid. In concentratedsulfuric acid, [TcO4]− converts to the octahedral form TcO3(OH)(H2O)2, the conjugate base of the hypothetical triaquo complex [TcO3(H2O)3]+.[42]
Technetium forms adioxide,[43]disulfide, diselenide, and ditelluride. An ill-defined Tc2S7 forms upon treatingpertechnate with hydrogen sulfide. It thermally decomposes into disulfide and elemental sulfur.[44] Similarly the dioxide can be produced by reduction of the Tc2O7.
Unlike the case for rhenium, a trioxide has not been isolated for technetium. However, TcO3 has been identified in the gas phase usingmass spectrometry.[45]
Technetium forms the complexTcH2− 9. The potassium salt isisostructural withReH2− 9.[46] At high pressure formation of TcH1.3 from elements was also reported.[33]
TcCl4 forms chain-like structures, similar to the behavior of several other metal tetrachlorides.
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. Theoxidation states 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.[47][48] 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.[48]
TcCl4 → α-TcCl3 + 1/2 Cl2
TcCl3 → β-TcCl2 + 1/2 Cl2
The structure of TcCl4 is composed of infinite zigzag chains of edge-sharing TcCl6 octahedra. It is isomorphous to transition metal tetrachlorides ofzirconium,hafnium, andplatinum.[48]
Chloro-containing coordination complexes of technetium (99Tc) in various oxidation states: Tc(III), Tc(IV), Tc(V), and Tc(VI) represented.
Two polymorphs oftechnetium trichloride exist, α- and β-TcCl3. The α polymorph is also denoted as Tc3Cl9. It adopts a confacialbioctahedral structure.[49] It is prepared by treating the chloro-acetate Tc2(O2CCH3)4Cl2 with HCl. LikeRe3Cl9, 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 formolybdenum trichloride. TcBr3 does not adopt the structure of either trichloride phase. Instead it has the structure ofmolybdenum 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 ofTiI3, featuring chains of confacial octahedra with equal Tc—Tc contacts.[48]
Several anionic technetium halides are known. The binary tetrahalides can be converted to the hexahalides [TcX6]2− (X = F, Cl, Br, I), which adoptoctahedral molecular geometry.[24] 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 twobridging ligand halogen atoms such aschlorine orbromine.[50]
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.[52] The binary carbonyl Tc2(CO)10 is a white volatile solid.[53] In this molecule, two technetium atoms are bound to each other; each atom is surrounded byoctahedra of five carbonyl ligands. The bond length between technetium atoms, 303 pm,[54][55] is significantly larger than the distance between two atoms in metallic technetium (272 pm). Similarcarbonyls are formed by technetium'scongeners, manganese and rhenium.[56] Interest in organotechnetium compounds has also been motivated by applications innuclear medicine.[52] Technetium also forms aquo-carbonyl complexes, one prominent complex being [Tc(CO)3(H2O)3]+, which are unusual compared to other metal carbonyls.[52]
Technetium, withatomic numberZ = 43, is the lowest-numbered element in the periodic table for which all isotopes areradioactive. The second-lightest exclusively radioactive element,promethium, has atomic number 61.[32]Atomic nuclei with an odd number ofprotons are less stable than those with even numbers, even when the total number ofnucleons (protons +neutrons) is even,[57] and odd numbered elements have fewer stableisotopes.
The most stableradioactive isotopes are technetium-97 with ahalf-life of4.21±0.16 million years and technetium-98 with4.2±0.3 million years; current measurements of their half-lives give overlappingconfidence intervals corresponding to onestandard 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.[1] Thirty-four other radioisotopes have been characterized withmass numbers ranging from 86 to 122.[1] Most of these have half-lives that are less than an hour, the exceptions being technetium-93 (2.73 hours), technetium-94 (4.88 hours), technetium-95 (20 hours), and technetium-96 (4.3 days).[58]
The primarydecay mode for isotopes lighter than technetium-98 (98Tc) iselectron capture, producingmolybdenum (Z = 42).[59] For technetium-98 and heavier isotopes, the primary mode isbeta emission (the emission of anelectron orpositron), producingruthenium (Z = 44), with the exception that technetium-100 can decay both by beta emission and electron capture.[59][60]
Technetium also has numerousnuclear isomers, which are isotopes with one or moreexcited nucleons. Technetium-97m (97mTc; "m" stands formetastability) is the most stable, with a half-life of 91 days andexcitation energy 0.0965 MeV.[58]This is followed by technetium-95m (61 days, 0.03 MeV), and technetium-99m (6.01 hours, 0.142 MeV).[58]
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. One gram of technetium-99 produces6.2 × 108 disintegrations per second (in other words, thespecific activity of99Tc is 0.62 GBq/g).[28]
Technetium occurs naturally in the Earth'scrust in minute concentrations of about 0.003 parts per trillion. Technetium is so rare because thehalf-lives of97Tc and98Tc are only4.2 million years. More than a thousand of such periods have passed since the formation of theEarth, so the probability of survival of even one atom ofprimordial technetium is effectively zero. However, small amounts exist as spontaneousfission products inuranium ores. A kilogram of uranium contains an estimated 1 nanogram(10−9 g), equivalent to ten trillion atoms, of technetium.[21][61][62]Somered giant stars with the spectral types S, M, and N display a spectral absorption line indicating the presence of technetium.[25][63] These red giants are known informally astechnetium stars.
In contrast to the rare natural occurrence, bulk quantities of technetium-99 are produced each year fromspent nuclear fuel rods, which contain various fission products. The fission of a gram ofuranium-235 innuclear reactors yields 27 mg of technetium-99, giving technetium afission product yield of 6.1%.[28] Otherfissile isotopes produce similar yields of technetium, such as 4.9% fromuranium-233 and 6.21% fromplutonium-239.[64] An estimated 49,000 TBq (78 metric tons) of technetium was produced in nuclear reactors between 1983 and 1994, by far the dominant source of terrestrial technetium.[65][66]Only a fraction of the production is used commercially.[c]
Technetium-99 is produced by thenuclear fission of both uranium-235 and plutonium-239. It is therefore present inradioactive waste and in thenuclear fallout offission bomb explosions. Its decay, measured inbecquerels per amount of spent fuel, is the dominant contributor to nuclear waste radioactivity after about104~106 years after the creation of the nuclear waste.[65] From 1945–1994, an estimated 160 TBq (about 250 kg) of technetium-99 was released into the environment during atmosphericnuclear tests.[65][67]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 bynuclear 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 theSellafield plant, which released an estimated 550 TBq (about 900 kg) from 1995 to 1999 into theIrish Sea.[66] From 2000 onwards the amount has been limited by regulation to 90 TBq (about 140 kg) per year.[68]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 westCumbria contain about 1 Bq/kg of technetium.[69][70][d]
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 majoractinides in conventionalnuclear reprocessing. The liquid left after plutonium–uranium extraction (PUREX) contains a high concentration of technetium asTcO− 4 but almost all of this is technetium-99, not technetium-99m.[72]
The vast majority of the technetium-99m used in medical work is produced by irradiating dedicatedhighly enriched uranium targets in a reactor, extracting molybdenum-99 from the targets in reprocessing facilities,[37] and recovering at the diagnostic center the technetium-99m produced upon decay of molybdenum-99.[73][74] Molybdenum-99 in the form of molybdateMoO2− 4 isadsorbed onto acid alumina (Al 2O 3) in ashieldedcolumn chromatograph inside atechnetium-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.[21] The solublepertechnetateTcO− 4 can then be chemically extracted byelution using asaline solution. A drawback of this process is that it requires targets containing uranium-235, which are subject to the security precautions of fissile materials.[75][76]
The long half-life of technetium-99 and its potential to formanionic species creates a major concern for long-termdisposal of radioactive waste. Many of the processes designed to remove fission products in reprocessing plants aim atcationic species such ascaesium (e.g.,caesium-137) andstrontium (e.g.,strontium-90). Hence the pertechnetate escapes through those processes. Current disposal options favorburial 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 andiodide tend not to adsorb into the surfaces of minerals, and are likely to be washed away. By comparisonplutonium,uranium, andcaesium tend to bind to soil particles. Technetium could be immobilized by some environments, such as microbial activity in lake bottom sediments,[78] and theenvironmental chemistry of technetium is an area of active research.[79]
An alternative disposal method,transmutation, has been demonstrated atCERN for technetium-99. In this process, the technetium (technetium-99 as a metal target) is bombarded withneutrons to form the short-lived technetium-100 (half-life = 16 seconds) which decays by beta decay to stableruthenium-100. If recovery of usable ruthenium is a goal, an extremely pure technetium target is needed; if small traces of theminor actinides such asamericium andcurium are present in the target, they are likely to undergo fission and form morefission 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.[80]
The actual separation of technetium-99 from spent nuclear fuel is a long process. Duringfuel 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.[81]
Molybdenum-99, which decays to form technetium-99m, can be formed by theneutron activation of molybdenum-98.[82] 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 ofruthenium-96).[83]
The feasibility of technetium-99m production with the 22-MeV-proton bombardment of a molybdenum-100 target in medical cyclotrons following the reaction100Mo(p,2n)99mTc was demonstrated in 1971.[84] The recent shortages of medical technetium-99m reignited the interest in its production by proton bombardment of isotopically enriched (>99.5%) molybdenum-100 targets.[85][86] Other techniques are being investigated for obtaining molybdenum-99 from molybdenum-100 via (n,2n) or (γ,n) reactions in particle accelerators.[87][88][89]
Technetium-99m ("m" indicates that this is ametastable nuclear isomer) is used in radioactive isotopemedical tests. For example, technetium-99m is aradioactive tracer that medical imaging equipment tracks in the human body.[21][85] It is well suited to the role because it emits readily detectable 140 keVgamma rays, and its half-life is 6.01 hours (meaning that about 94% of it decays to technetium-99 in 24 hours).[28] The chemistry of technetium allows it to be bound to a variety of biochemical compounds, each of which determines how it is metabolized and deposited in the body, and this single isotope can be used for a multitude of diagnostic tests. More than 50 commonradiopharmaceuticals are based on technetium-99m for imaging and functional studies of thebrain, heart muscle,thyroid,lungs,liver,gall bladder,kidneys,skeleton,blood, andtumors.[90]
The longer-lived isotope, technetium-95m with a half-life of 61 days, is used as aradioactive tracer to study the movement of technetium in the environment and in plant and animal systems.[91]
Technetium-99 decays almost entirely by beta decay, emitting beta particles with consistent low energies and no accompanying gamma rays. Moreover, its 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 U.S.National Institute of Standards and Technology (NIST) standard beta emitter, and is used for equipment calibration.[92] Technetium-99 has also been proposed for optoelectronic devices andnanoscalenuclear batteries.[93]
Likerhenium andpalladium, technetium can serve as acatalyst. In processes such as thedehydrogenation ofisopropyl alcohol, it is a far more effective catalyst than either rhenium or palladium. However, its radioactivity is a major problem in safe catalytic applications.[94]
When steel is immersed in water, adding a small concentration (55 ppm) of potassium pertechnetate(VII) to the water protects thesteel from corrosion,[95] even if the temperature is raised to 250 °C (523 K).[96] For this reason, pertechnetate has been used as an anodiccorrosion inhibitor for steel, although technetium's radioactivity poses problems that limit this application to self-contained systems.[97] While (for example)CrO2− 4 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.[96] 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 oftechnetium 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.[98]
As noted, the radioactive nature of technetium (3 MBq/L at the concentrations required) makes this corrosion protection impractical in almost all situations.[95] Nevertheless, corrosion protection by pertechnetate ions was proposed (but never adopted) for use inboiling water reactors.[98]
Technetium plays no natural biological role and is not normally found in the human body.[25] Technetium is produced in quantity by nuclear fission, and spreads more readily than many radionuclides. It appears to have low chemical toxicity. For example, no significant change in blood formula, body and organ weights, and food consumption could be detected for rats which ingested up to 15 μg of technetium-99 per gram of food for several weeks.[99] In the body, technetium quickly gets converted to the stableTcO− 4 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.[100]
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; suchradioactive contamination in the lungs can pose a significant cancer risk. For most work, careful handling in afume hood is sufficient, and aglove box is not needed.[101]
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.[102]
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^Irregular crystals and trace impurities raise this transition temperature to 11.2 K for 99.9% pure technetium powder.[29]
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