Technetium
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, 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. In 1937, technetium became the first predominantly artificial element to be produced, hence its name, 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 and ruthenium. 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 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.| 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
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. This name caused significant resentment in the scientific community, because it was interpreted as referring to a series of victories 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. The group bombarded columbite with a beam of electrons and deduced element 43 was present by examining X-ray emission 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. Later experimenters could not replicate the discovery, and it was dismissed as an error. 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 panormium, after the Latin name for Palermo, Panormus. 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 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, 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.
Characteristics
Physical properties
Technetium is a silvery-gray radioactive metal with an appearance similar to platinum, commonly obtained as a gray powder. The crystal structure of the bulk pure metal is hexagonal close-packed. Atomic technetium has characteristic emission lines at wavelengths of 363.3 nm, 403.1 nm, 426.2 nm, 429.7 nm, and 485.3 nm. The unit cell parameters of the orthorhombic Tc metal were reported when Tc is contaminated with carbon, = 0.4958, = 0.4474·nm for Tc-C with 1.38 wt% C and = 0.2815, = 0.4963, = 0.4482. The metal form is slightly paramagnetic, meaning its magnetic dipoles align with external magnetic fields, but will assume random orientations once the field is removed. 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. Technetium exhibits nine oxidation states from −1 to +7, with +4, +5, and +7 being the most common. Technetium dissolves in aqua regia, nitric acid, and concentrated sulfuric acid, but not in hydrochloric acid of any concentration.Metallic technetium slowly tarnishes 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.
Compounds
Pertechnetate and other derivatives
The most prevalent form of technetium that is easily accessible is sodium pertechnetate, Na. The majority of this material is produced by radioactive decay from 2−:Pertechnetate is only weakly hydrated in aqueous solutions 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:
HTcO4 is a strong acid. In concentrated sulfuric acid, − converts to the octahedral form TcO32, the conjugate base of the hypothetical triaquo complex +.
Other chalcogenide derivatives
Technetium forms a dioxide, disulfide, diselenide, and ditelluride. An ill-defined Tc2S7 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 Tc2O7.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.