Tungsten is a chemical element; it has symbol W (from )"wolfram" on Merriam-Webster."wolfram" on Oxford Dictionaries. and atomic number 74. It is a metal found naturally on Earth almost exclusively in compounds with other elements. It was identified as a distinct element in 1781 and first isolated as a metal in 1783. Its important ores include scheelite and wolframite, the latter lending the element its alternative name.
The free element is remarkable for its robustness, especially the fact that it has the highest melting point of all known elements, at . It also has the highest boiling point, at . Its density is 19.254 g/cm3, comparable with that of uranium and gold, and much higher (about 1.7 times) than that of lead. Polycrystalline tungsten is an intrinsically brittle and hard material (under standard conditions, when uncombined), making it difficult to work into metal. However, pure single-crystalline tungsten is more ductile and can be cut with a hard-steel hacksaw.
Tungsten occurs in many alloys, which have numerous applications, including incandescent light bulb filaments, X-ray tubes, electrodes in gas tungsten arc welding, superalloys, and radiation shielding. Tungsten's hardness and high density make it suitable for military applications in penetrating projectiles. Tungsten compounds are often used as industrial catalysts. Its largest use is in tungsten carbide, a wear-resistant material used in metalworking, mining, and construction. About 50% of tungsten is used in tungsten carbide, with the remaining major use being alloys and steels: less than 10% is used in other compounds.
Tungsten is the only metal in the third transition series that is known to occur in biomolecules, found in a few species of bacteria and archaea. However, tungsten interferes with molybdenum and copper metabolism and is somewhat toxic to most forms of animal life.
Characteristics
Physical properties
In its raw form, tungsten is a hard steel-grey metal that is often brittle and hard to work. Purified, monocrystalline tungsten retains its hardness (which exceeds that of many steels), and becomes malleable enough that it can be worked easily. It can be worked by forging, drawing, rolling, or extruding. Most materials based on tungsten are formed by sintering tungsten powder along with additives, though this results in a more porous final product.
Of all metals in pure form, tungsten has the highest melting point (), lowest vapor pressure (at temperatures above ), and the highest tensile strength. Although carbon remains solid at higher temperatures than tungsten, carbon sublimes at atmospheric pressure instead of melting, so it has no melting point. Moreover, tungsten's most stable crystal phase does not exhibit any high-pressure-induced structural transformations for pressures up to at least 364 gigapascals. Tungsten has the lowest coefficient of thermal expansion of any pure metal. The low thermal expansion and high melting point and tensile strength of tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d electrons. Alloying small quantities of tungsten with steel greatly increases its toughness.
Tungsten exists in two major crystalline forms: α and β. The former has a body-centered cubic structure and is the more stable form. The structure of the β phase is called A15 cubic; it is metastable, but can coexist with the α phase at ambient conditions owing to non-equilibrium synthesis or stabilization by impurities. Contrary to the α phase which crystallizes in isometric grains, the β form exhibits a columnar habit. The α phase has one third of the electrical resistivityBean, Heather (October 19, 1998). Material Properties and Analysis Techniques for Tungsten Thin Films. frii.com and a much lower superconducting transition temperature Tc relative to the β phase: ca. 0.015 K vs. 1–4 K; mixing the two phases allows obtaining intermediate Tc values. The Tc value can also be raised by alloying tungsten with another metal (e.g. 7.9 K for W-Tc). Such tungsten alloys are sometimes used in low-temperature superconducting circuits.
Isotopes
Naturally occurring tungsten consists of four stable isotopes (W, W, W, and W) and one very long-lived radioisotope, W. Theoretically, all five can decay into isotopes of element 72 (hafnium) by alpha emission, but only W has been observed to do so, with a half-life of years; on average, this yields about two alpha decays of W per gram of natural tungsten per year.
Another 34 artificial radioisotopes of tungsten have been characterized, the most stable of which are W with a half-life of 121.2 days, W with a half-life of 75.1 days, W with a half-life of 69.4 days, W with a half-life of 21.6 days, and W with a half-life of 23.72 h. All of the remaining radioactive isotopes have half-lives of less than 3 hours, and most of these have half-lives below 8 minutes. Tungsten also has 12 meta states, with the most stable being W (t½ 6.4 minutes).
Chemical properties Tungsten is a mostly non-reactive element: it does not react with water, is immune to attack by most acids and bases, and does not react with oxygen or air at room temperature. At elevated temperatures (i.e., when red-hot) it reacts with oxygen to form the trioxide compound tungsten(VI), . It will, however, react directly with fluorine () at room temperature to form tungsten(VI) fluoride (), a colorless gas. At around 250 °C it will react with chlorine or bromine, and under certain hot conditions will react with iodine. Finely divided tungsten is pyrophoric.
The most common formal oxidation state of tungsten is +6, but it exhibits all oxidation states from −2 to +6. Tungsten typically combines with oxygen to form the yellow tungstic oxide, , which dissolves in aqueous alkaline solutions to form tungstate ions, .
Tungsten carbides ( and WC) are produced by heating powdered tungsten with carbon. is resistant to chemical attack, although it reacts strongly with chlorine to form tungsten hexachloride ().
In aqueous solution, tungstate gives the heteropoly acids and polyoxometalate anions under neutral and acidic conditions. As tungstate is progressively treated with acid, it first yields the soluble, metastable "paratungstate A" anion, , which over time converts to the less soluble "paratungstate B" anion, . Further acidification produces the very soluble metatungstate anion, , after which equilibrium is reached. The metatungstate ion exists as a symmetric cluster of twelve tungsten-oxygen octahedra known as the Keggin anion. Many other polyoxometalate anions exist as metastable species. The inclusion of a different atom such as phosphorus in place of the two central hydrogens in metatungstate produces a wide variety of heteropoly acids, such as phosphotungstic acid .
Tungsten trioxide can form intercalation compounds with alkali metals. These are known as bronzes, and have the general formula of MWO, where M is an alkali metal; an example is sodium tungsten bronze. Tungsten trioxide is also intercalated with hydrogen ions in electrochromic devices, that is, devices that store a charge and change color.
In gaseous form, tungsten forms the diatomic species . These molecules feature a sextuple bond between tungsten atoms — the highest known bond order among stable atoms.
History In 1781, Carl Wilhelm Scheele discovered that a new acid, tungstic acid, could be made from scheelite (at the time called tungsten).Scheele, Carl Wilhelm (1781) "Tungstens bestånds-delar" (Tungsten's constituents), Kungliga Vetenskaps Academiens Nya Handlingar (Royal Scientific Academy's New Proceedings), 2 : 89–95 (in Swedish).English translation on pp. 4–13 of: de Luyart, John Joseph and Fausto, with Charles Cullen, trans., A Chemical Analysis of Wolfram and Examination of a New Metal, Which Enters its Composition (London, England, G. Nicol, 1785). Scheele and Torbern Bergman suggested that it might be possible to obtain a new metal by reducing this acid. In 1783, José and Fausto Elhuyar found an acid made from wolframite that was identical to tungstic acid. Later that year, at the Royal Basque Society in the town of Bergara, Spain, the brothers succeeded in isolating tungsten by reduction of this acid with charcoal, and they are credited with the discovery of the element (they called it "wolfram" or "volfram").de Luyart, J.J. and F. (September 1783) "Análisis químico del volfram, y examen de un nuevo metal, que entra en su composición" (Chemical analysis of wolframite, and examination of a new metal, which enters into its composition), Extractos de las Juntas Generales celebradas por la Real Sociedad Bascongada de los Amigos del País en la ciudad de Vitoria por setiembre de 1783, pp. 46–88.de Luyart, John Joseph and Fausto, with Charles Cullen, trans., A Chemical Analysis of Wolfram and Examination of a New Metal, Which Enters its Composition (London, England, G. Nicol, 1785).Caswell, Lyman R. and Stone Daley, Rebecca W. (1999) "The Delhuyar brothers, tungsten, and Spanish silver," Bulletin for the History of Chemistry, 23 : 11–19. Available at: University of Illinois (USA)
The strategic value of tungsten came to notice in the early 20th century. British authorities acted in 1912 to free the Carrock mine from the German owned Cumbrian Mining Company and, during World War I, restrict German access elsewhere. In World War II, tungsten played a more significant role in background political dealings. Portugal, as the main European source of the element, was put under pressure from both sides because of its deposits of wolframite ore at Panasqueira. Tungsten's desirable properties such as resistance to high temperatures, its hardness and density, and its strengthening of alloys made it an important raw material for the arms industry, both as a constituent of weapons and equipment and employed in production itself, e.g., in tungsten carbide cutting tools for machining steel. Elemental tungsten is mostly used in the 21st century to make the alloy tungsten steel, but it also finds uses in the filaments of incandescent lamps as well as turbine blades.
Tungsten is unique amongst the elements in that it has been the subject of patent proceedings. In 1928, a US court rejected General Electric's attempt to patent it, overturning granted in 1913 to William D. Coolidge.General Electric Co. v. De Forest Radio Co., 28 F.2d 641, 643 (3rd Cir. 1928)
It is suggested that remnants of wolfram have been found in what may have been the garden of the astronomer and alchemist Tycho Brahe.
Etymology The name tungsten (which means in Swedish and was the old Swedish name for the mineral scheelite and other minerals of similar density) is used in English, French, and many other languages as the name of the element, but wolfram (or volfram) is used in most European (especially Germanic and Slavic) languages and is derived from the mineral wolframite, which is the origin of the chemical symbol W. The name wolframite is derived from German (), the name given to tungsten by Johan Gottschalk Wallerius in 1747. This, in turn, derives from Latin , the name Georg Agricola used for the mineral in 1546, which translates into English as and is a reference to the large amounts of tin consumed by the mineral during its extraction, as though the mineral devoured it like a wolf.
The name wolfram was accepted by the International Union of Pure and Applied Chemistry in 1949; this decision was disputed by English-speaking scientists, and the next year it was revised to reflect the widespread use of the name tungsten in that region, though the symbol W was kept. In Germany and several other European countries, the name wolfram is still preferred, while tungsten remains more common in Britain and the United States.
Occurrence