Tungsten
Tungsten is a chemical element which has the symbol W 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 than that of lead. Polycrystalline tungsten is an intrinsically brittle and hard material, 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, and becomes malleable enough that it can be worked easily. It can be worked by forging, drawing, or extruding, but is more commonly formed by sintering. Sintering is often used due to the very high melting point of tungsten.Of all metals in pure form, tungsten has the highest melting point, lowest vapor pressure, 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 resistivity 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. Such tungsten alloys are sometimes used in low-temperature superconducting circuits.
Isotopes
Naturally occurring tungsten consists of four stable isotopes and one very long-lived radioisotope, 180W. Theoretically, all five can decay into isotopes of element 72 by alpha emission, but only 180W has been observed to do so, with a half-life of years; on average, this yields about two alpha decays of 180W per gram of natural tungsten per year. This rate is equivalent to a specific activity of roughly 63 micro-becquerel per kilogram. This rate of decay is orders of magnitude lower than that observed in carbon or potassium as found on earth, which likewise contain small amounts of long-lived radioactive isotopes. Bismuth was long thought to be non-radioactive, but actually decays with a half-life of years or about a factor 10 slower than. However, due to naturally occurring bismuth being 100%, its specific activity is actually higher than that of natural tungsten at 3 milli-becquerel per kilogram. The other naturally occurring isotopes of tungsten have not been observed to decay, constraining their half-lives to be at least.Another 34 artificial radioisotopes of tungsten have been characterized, the most stable of which are 181W with a half-life of 121.2 days, 185W with a half-life of 75.1 days, 188W with a half-life of 69.4 days, 178W with a half-life of 21.6 days, and 187W 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 179mW.
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 it reacts with oxygen to form the trioxide compound tungsten, WO3. It will, however, react directly with fluorine at room temperature to form tungsten 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, WO3, which dissolves in aqueous alkaline solutions to form tungstate ions,.
Tungsten carbides are produced by heating powdered tungsten with carbon. W2C 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 H3PW12O40.
Tungsten trioxide can form intercalation compounds with alkali metals. These are known as bronzes; an example is sodium tungsten bronze.
In gaseous form, tungsten forms the diatomic species W2. 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. 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.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.
Now tungsten is used in many more applications such as aircraft and motorsport ballast weights, darts, anti-vibration tooling, and sporting equipment.
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.
It is suggested that remnants of wolfram have been found in what may have been the garden of the astronomer and alchemist Tycho Brahe.