"Group 6" is the new IUPAC name for this group; the old style name was "group VIB" in the old US system (CAS) or "group VIA" in the European system (old IUPAC). Group 6 must not be confused with the group with the old-style group crossed names of eitherVIA (US system, CAS) orVIB (European system, old IUPAC).That group is now calledgroup 16.
Molybdenite—the principal ore from which molybdenum is now extracted—was previously known as molybdena, which was confused with and often implemented as though it weregraphite. Like graphite, molybdenite can be used to blacken a surface or as a solid lubricant.[4] Even when molybdena was distinguishable from graphite, it was still confused with agalena (a common lead ore), which took its name fromAncient GreekΜόλυβδοςmolybdos, meaninglead.[5] It was not until 1778 thatSwedish chemistCarl Wilhelm Scheele realized that molybdena was neither graphite nor lead.[6][7] He and other chemists then correctly assumed that it was the ore of a distinct new element, namedmolybdenum for the mineral in which it was discovered.Peter Jacob Hjelm successfully isolated molybdenum by usingcarbon andlinseed oil in 1781.[5][8]
Regarding tungsten, in 1781Carl Wilhelm Scheele discovered that a newacid,tungstic acid, could be made fromscheelite (at the time named tungsten). Scheele andTorbern Bergman suggested that it might be possible to obtain a new metal by reducing this acid.[9] In 1783,José andFausto Elhuyar found an acid made from wolframite that was identical to tungstic acid. Later that year, inSpain, the brothers succeeded in isolating tungsten by reduction of this acid withcharcoal, and they are credited with the discovery of the element.[10][11]
Seaborgium was first produced by a team of scientists led by Albert Ghiorso who worked at the Lawrence Berkeley Laboratory in Berkeley, California, in 1974. They created seaborgium by bombarding atoms of californium-249 with ions of oxygen-18 until seaborgium-263 was produced.
During the 1800s, chromium was primarily used as a component of paints and intanning salts. At first, crocoite fromRussia was the main source, but in 1827, a larger chromite deposit was discovered nearBaltimore,United States. This made the United States the largest producer of chromium products until 1848 when large deposits of chromite where found nearBursa,Turkey.[12] Chromium was used for electroplating as early as 1848, but this use only became widespread with the development of an improved process in 1924.[13]
For about a century after its isolation, molybdenum had no industrial use, owing to its relative scarcity, difficulty extracting the pure metal, and the immaturity of the metallurgical subfield.[14][15][16] Early molybdenum steel alloys showed great promise in their increased hardness, but efforts were hampered by inconsistent results and a tendency toward brittleness and recrystallization. In 1906,William D. Coolidge filed a patent for rendering molybdenumductile, leading to its use as a heating element for high-temperature furnaces and as a support for tungsten-filament light bulbs; oxide formation and degradation require that moly be physically sealed or held in an inert gas. In 1913,Frank E. Elmore developed aflotation process to recovermolybdenite from ores; flotation remains the primary isolation process. During thefirst World War, demand for molybdenum spiked; it was used both inarmor plating and as a substitute for tungsten inhigh-speed steels. Some British tanks were protected by 75 mm (3 in)manganese steel plating, but this proved to be ineffective. The manganese steel plates were replaced with 25 mm (1 in) molybdenum-steel plating allowing for higher speed, greater maneuverability, and better protection.[5] After the war, demand plummeted until metallurgical advances allowed extensive development of peacetime applications. InWorld War II, molybdenum again saw strategic importance as a substitute for tungsten in steel alloys.[17]
InWorld War II, tungsten played a 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 ofwolframite ore atPanasqueira. Tungsten's resistance to high temperatures and its strengthening of alloys made it an important raw material for the arms industry.[18]
Unlike other groups, the members of this family do not show patterns in itselectron configuration, as two lighter members of the group are exceptions from theAufbau principle:
Most of the chemistry has been observed only for the first three members of the group. The chemistry of seaborgium is not very established and therefore the rest of the section deals only with its upper neighbors in theperiodic table. The elements in the group, like those of groups 7–11, have high melting points, and form volatile compounds in higheroxidation states. All the elements of the group are relatively nonreactive metals with a high melting points (1907 °C, 2477 °C, 3422 °C); that of tungsten is the highest of all metals. The metals form compounds in different oxidation states: chromium forms compounds in all states from −2 to +6:[19] disodium pentacarbonylchromate, disodium decacarbonyldichromate,bis(benzene)chromium, tripotassium pentanitrocyanochromate,chromium(II) chloride,chromium(III) oxide,chromium(IV) chloride,potassium tetraperoxochromate(V), andchromium(VI) dichloride dioxide; the same is also true for molybdenum and tungsten, but the stability of the +6 state grows down the group.[19] Depending on oxidation states, the compounds are basic, amphoteric, or acidic; the acidity grows with the oxidation state of the metal.
The two main products of chromium refinement are ferrochromium and chromium metal. Ferrochromium is obtained by reducing chromite ore with eitheraluminium orsilicon athigh temperatures.[21] For chromium metal, the iron must be separated first using a two-step roasting and leaching process. By heating with sodium and calcium carbonates in air, the chromium is oxidised to its hexavalent form, while the iron forms stable Fe2O3. The insoluble iron oxide is removed in the subsequent leaching process, and the resulting chromium can be isolated by reaction with sulfuric acid (to form the dichromate), reduction with carbon (to form Cr2O3), and finally reduction with aluminium.[21]
Molybdenum is the 54th most abundant element in the Earth's crust, and 42nd most abundant in the Universe.[22][23] It is refined mainly frommolybdenite, but is also present inwulfenite (PbMoO4) andpowellite (CaMoO4). It is both mined as a principal ore and is also recovered as a byproduct of copper and tungsten mining.[24] It is mainly mined in the United States, China, Chile, and Peru, with the total amount produced being 200,000 tonnes per year.[25]
The processing of molybdenite involves roasting in air at 700 °C to produce MoO3, which is then usually extracted with aqueous ammonia to give ammonium molybdate. Copper impurities are then removed by treatment with hydrogen sulfide.[26] Ammonium molybdate converts toammonium dimolybdate, which is isolated as a solid and heated to produce molybdenum trioxide.[27] Reduction of this oxide with hydrogen affords molybdenum metal.[28]
Tungsten is slightly more abundant in the Earth's crust than molybdenum, with an average concentration of 1.25 ppm.[29] It is mainly found in the mineralswolframite andscheelite, and it usually never occurs as a free element in nature.[30] The largest producers of tungsten in the world areChina (1,800,000 t),Canada (290,000 t),Russia (160,000 t),Vietnam (95,000 t) andBolivia.[31]
Tungsten is extracted from its ore through a multi-step process, resulting in WO3. This is then reduced with hydrogen or carbon to produce powdered tungsten.[32] Due to tungsten's very high melting point, it is often sintered with another metal, such as nickel, to produce an alloy with a lower melting point.[33]
Seaborgium is a synthetictransuranium element that does not occur in nature. The seaborgium-260 isotope can be made by bombarding lead-207 or -208 with chromium-54 nuclei;[34] seaborgium-263 by bombardingcalifornium-249 withoxygen-18 nuclei;[35] and seaborgium-267 by bombarding hassium-271 with α-particles,[36] among others.
While trivalent chromium compounds and chromium metal are insoluble, and are not considered a health hazard,hexavalent chromium compounds aregenotoxiccarcinogens.[37][38] The acute oral toxicity of hexavalent chromium is 1.5-3.3 mg/kg,[39] as it enters cells through similar transport mechanisms assulfate andphosphate. Its strongoxidizing capability causes damage to the kidneys, liver, and red blood cells, resulting in liver and renal failure and hemolysis.[40] Chromate salts can also cause allergic reactions, leading to contact dermatitis, irritant dermatitis, and ulceration, sometimes known as "chrome ulcers".[41][42]
Molybdenum is an essential element in most organisms, and an essential trace dietary element in humans.[43] However, long-term exposure to molybdenum dust and fumes can be toxic, causing irritation to the eyes and skin at low levels and fatigue, headaches, and joint pain at high levels.[44]
Group 6 is notable in that it contains some of the only elements in periods 5 and 6 with a known role in the biological chemistry of living organisms: molybdenum is common inenzymes of many organisms including humans, andtungsten has been identified in an analogous role in enzymes from somearchaea, such asPyrococcus furiosus. In contrast, and unusually for a first-row d-block transition metal, chromium appears to have few biological roles, although it is thought to form part of theglucose metabolism enzyme in some mammals.
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^National Research Council (U.S.). Committee on Biologic Effects of Atmospheric Pollutants (1974).Chromium. National Academy of Sciences. p. 155.ISBN978-0-309-02217-0.
^Dennis, J. K.; Such, T. E. (1993). "History of Chromium Plating".Nickel and Chromium Plating. Woodhead Publishing. pp. 9–12.ISBN978-1-85573-081-6.
^Hoyt, Samuel Leslie (1921).Metallography, Volume 2. McGraw-Hill.
^Krupp, Alfred; Wildberger, Andreas (1888).The metallic alloys: A practical guide for the manufacture of all kinds of alloys, amalgams, and solders, used by metal-workers ... with an appendix on the coloring of alloys. H.C. Baird & Co. p. 60.
^Gupta, C.K. (1992).Extractive Metallurgy of Molybdenum. CRC Press.ISBN978-0-8493-4758-0.
^Millholland, Ray (August 1941). "Battle of the Billions: American industry mobilizes machines, materials, and men for a job as big as digging 40 Panama Canals in one year".Popular Science. p. 61.
^Stevens, Donald G. (1999). "World War II Economic Warfare: The United States, Britain, and Portuguese Wolfram".The Historian.61 (3):539–556.doi:10.1111/j.1540-6563.1999.tb01036.x.
^abSchmidt, Max (1968). "VI. Nebengruppe".Anorganische Chemie II (in German). Wissenschaftsverlag. pp. 119–127.
^Emsley, John (2002).Nature's building blocks: an A - Z guide to the elements (Repr. (with corr.) ed.). Oxford: Oxford University Press.ISBN978-0-19-850341-5.
^Considine, Glenn D., ed. (1984).Van Nostrand's Encyclopedia of Chemistry (5 ed.). Hoboken, NJ: Wiley-Interscience. pp. 1038–1040.ISBN978-0-471-61525-5.
^Lide, David R.; Chemical Rubber Company, eds. (1993).CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data (74. ed., 1993-1994 ed.). Boca Raton, Fla.: CRC Press.ISBN978-0-8493-0474-3.
^Holleman, Arnold F.; Nils, Wiberg (2019). Wiberg, Egon (ed.).Lehrbuch der anorganischen Chemie (91.–100. Aufl. Reprint 2019 ed.). Berlin Boston: De Gruyter.ISBN978-3-11-007511-3.
^Ullmann's encyclopedia of industrial chemistry. Weinheim: Wiley-VCH. 2010.ISBN978-3-527-30673-2.
^Gupta, C. K. (1992).Extractive metallurgy of molybdenum. Boca Raton: CRC Press.ISBN978-0-8493-4758-0.
^"Abundance of Elements in the Earth's Crust and in the Sea",CRC Handbook of Chemistry and Physics, 97th edition (2016–2017), sec. 14, pg. 17
^"Tungsten, W, atomic number 74".Institute of rare earths elements and strategic metals.
