View source for Group 6 element

{{short description|Group of chemical elements}}
{{Infobox periodic table group
| title       = Group&nbsp;6 {{nowrap|in the periodic table}}
| group number= 6
| trivial name=
| by element  = chromium group
| CAS         = VIB
| old IUPAC   = VIA
| mark        = Cr,Mo,W,Sg
| left        = [[Group 5 element|group 5]]
| right       = [[Group 7 element|group 7]]
}}
{| class="floatright"
! colspan=2 style="text-align:left;" | ↓&nbsp;<small>[[Period (periodic table)|Period]]</small>
|-
! [[Period 4 element|4]]
| {{element cell image|24|Chromium|Cr| |Solid|Transition metal|Primordial|image=Chromium crystals and 1cm3 cube.jpg|image caption=Chromium crystal bar}}
|-
! [[Period 5 element|5]]
| {{element cell image|42|Molybdenum |Mo| |Solid|Transition metal|Primordial|image=Molybdenum crystaline fragment and 1cm3 cube.jpg|image caption=Molybdenum crystal bar}}
|-
! [[Period 6 element|6]]
| {{element cell image|74|Tungsten |W| |Solid|Transition metal|Primordial|image=Wolfram evaporated crystals and 1cm3 cube.jpg|image caption=Tungsten crystal bar}}
|-
! [[Period 7 element|7]]
| {{element cell image|106|Seaborgium|Sg| |Unknown phase|Transition metal|Synthetic}}
|-
| colspan="2"|
----
''Legend''
{| style="text-align:center; border:0; margin: 0 auto"
|-
| style="border:{{element color|Primordial}}; background:{{Element color|table mark}}; padding:0 2px;" | [[primordial element]]
|-
| style="border:{{element color|Synthetic}}; background:{{Element color|table mark}}; padding:0 2px;" | [[synthetic element]]
|}
|}

'''Group 6''', numbered by [[IUPAC]] style, is a group of [[Chemical element|elements]] in the [[periodic table]]. Its members are [[chromium]] (Cr), [[molybdenum]] (Mo), [[tungsten]] (W), and [[seaborgium]] (Sg). These are all [[transition metal]]s and chromium, molybdenum and tungsten are [[refractory metals]].

The [[electron configuration]] of these elements do not follow a unified trend, though the outermost shells do correlate with trends in chemical behavior:

{| class="wikitable" style="white-space:nowrap;"
|-
!''[[Atomic number|Z]]'' !! [[Chemical element|Element]] !! Electrons per [[Electron shell|shell]]
|-
| 24 || chromium || 2, 8, 13, 1
|-
| 42 || molybdenum || 2, 8, 18, 13, 1
|-
| 74 || tungsten || 2, 8, 18, 32, 12, 2
|-
| 106 || seaborgium || 2, 8, 18, 32, 32, 12, 2
|}

"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 either ''VIA'' (US system, CAS) or ''VIB'' (European system, old IUPAC). ''That'' group is now called [[Group 16 element|group 16]].
<!--==Chemistry==
==Applications==-->

==History==
[[File:Cut Ruby.jpg|left|thumb|upright|The red colour of [[Ruby|rubies]] is from a small amount of chromium(III).]]

===Discoveries===
Chromium was first reported on July 26, 1761, when [[Johann Gottlob Lehmann (scientist)|Johann Gottlob Lehmann]] found an orange-red mineral in the [[Beryozovskoye deposit|Beryozovskoye mines]] in the [[Ural Mountains]] of [[Russia]], which he named "Siberian red lead," which was found out in less than 10 years to be a bright [[yellow]] pigment.<ref name="ChromiumVI"/> Though misidentified as a [[lead]] compound with [[selenium]] and [[iron]] components, the mineral was [[crocoite]] with a formula of [[lead chromate|PbCrO<sub>4</sub>]].<ref name="ChromiumVI">{{cite book|title = Chromium (VI) Handbook|publisher = CRC Press|year = 2005|isbn = 978-1-56670-608-7|pages = 7–11|author1=Guertin, Jacques |author2=Jacobs, James Alan |author3=Avakian, Cynthia P. }}</ref> Studying the mineral in 1797, [[Louis Nicolas Vauquelin]] produced [[chromium(VI) oxide|chromium trioxide]] by mixing crocoite with [[hydrochloric acid]], and metallic chromium by heating the oxide in a charcoal oven a year later.<ref>{{cite journal|url = https://books.google.com/books?id=6dgPAAAAQAAJ|journal =Journal of Natural Philosophy, Chemistry, and the Arts|year = 1798|page = 146|volume =3|title = Memoir on a New Metallic Acid which exists in the Red Lead of Sibiria|first = Louis Nicolas|last = Vauquelin}}</ref> He was also able to detect traces of chromium in precious [[gemstone]]s, such as [[ruby]] or [[emerald]].<ref name="ChromiumVI"/><ref>{{Cite web|last = van der Krogt|first = Peter|title = Chromium|url = http://elements.vanderkrogt.net/element.php?sym=Cr|access-date = 2008-08-24}}</ref>

[[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 were [[graphite]]. Like graphite, molybdenite can be used to blacken a surface or as a solid lubricant.<ref name="Lansdown1999">{{cite book | last1 = Lansdown | first1 = A.R. | title = Molybdenum disulphide lubrication | volume = 35 | work = Tribology and Interface Engineering | publisher = Elsevier | year = 1999 | isbn = 978-0-444-50032-8}}</ref> Even when molybdena was distinguishable from graphite, it was still confused with a [[galena]] (a common lead ore), which took its name from [[Ancient Greek]] {{lang|grc|Μόλυβδος}} ''{{lang|grc-Latn|molybdos}}'', meaning ''lead''.<ref name="nbb">{{cite book|last = Emsley|first = John|title = Nature's Building Blocks|url=https://books.google.com/books?id=j-Xu07p3cKwC&pg=PA265|publisher = Oxford University Press|year= 2001|location = Oxford|pages = 262–266|isbn =0-19-850341-5}}</ref> It was not until 1778 that [[Sweden|Swedish]] chemist [[Carl Wilhelm Scheele]] realized that molybdena was neither graphite nor lead.<ref name="elemental">{{cite web|last = Gagnon|first = Steve|title = Molybdenum|publisher = Jefferson Science Associates, LLC|url = http://education.jlab.org/itselemental/ele042.html|access-date = 2007-05-06}}</ref><ref>{{cite journal|author = Scheele, C. W. K.|title = Versuche mit Wasserbley;Molybdaena|journal = Svenska Vetensk. Academ. Handlingar|page=238|year = 1779|volume = 40|url = http://resolver.sub.uni-goettingen.de/purl?PPN324352840_0040}}</ref> He and other chemists then correctly assumed that it was the ore of a distinct new element, named ''molybdenum'' for the mineral in which it was discovered. [[Peter Jacob Hjelm]] successfully isolated molybdenum by using [[carbon]] and [[linseed oil]] in 1781.<ref name="nbb" /><ref>{{cite journal|author = Hjelm, P. J.|title = Versuche mit Molybdäna, und Reduction der selben Erde|journal = Svenska Vetensk. Academ. Handlingar|page = 268|year = 1788|volume = 49|url = http://resolver.sub.uni-goettingen.de/purl?PPN324352840_0009_02_NS}}</ref>

Regarding tungsten, in 1781 [[Carl Wilhelm Scheele]] discovered that a new [[acid]], [[tungstic acid]], could be made from [[scheelite]] (at the time named tungsten). Scheele and [[Torbern Bergman]] suggested that it might be possible to obtain a new metal by reducing this acid.<ref name="SaundersN">{{cite book|last=Saunders|first=Nigel|title=Tungsten and the Elements of Groups 3 to 7 (The Periodic Table)|publisher=Heinemann Library|location=[[Chicago, Illinois]]|date=February 2004|isbn=1-4034-3518-9|url-access=registration|url=https://archive.org/details/tungstenelements00nige}}</ref> In 1783, [[José Elhuyar|José]] and [[Fausto Elhuyar]] found an acid made from wolframite that was identical to tungstic acid. Later that year, in [[Spain]], the brothers succeeded in isolating tungsten by reduction of this acid with [[charcoal]], and they are credited with the discovery of the element.<ref name="ITIAnews_0605">{{cite news|url=http://www.itia.info/FileLib/Newsletter_2005_06.pdf|title=ITIA Newsletter|date=June 2005|publisher=International Tungsten Industry Association|access-date=2008-06-18|archive-url=https://web.archive.org/web/20110721214335/http://www.itia.info/FileLib/Newsletter_2005_06.pdf|archive-date=2011-07-21}}</ref><ref name="ITIAnews_1205">{{cite news|url=http://www.itia.info/FileLib/Newsletter_2005_12.pdf|title=ITIA Newsletter|date=December 2005|publisher=International Tungsten Industry Association|access-date=2008-06-18|archive-url=https://web.archive.org/web/20110721214335/http://www.itia.info/FileLib/Newsletter_2005_12.pdf|archive-date=2011-07-21}}</ref>

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.

===Historical development and uses===
During the 1800s, chromium was primarily used as a component of paints and in [[Tanning (leather)|tanning]] salts. At first, crocoite from [[Russia]] was the main source, but in 1827, a larger chromite deposit was discovered near [[Baltimore]], [[United States]]. This made the United States the largest producer of chromium products until 1848 when large deposits of chromite where found near [[Bursa]], [[Turkey]].<ref name="NRC">{{cite book|title = Chromium|author = National Research Council (U.S.). Committee on Biologic Effects of Atmospheric Pollutants|publisher = National Academy of Sciences|year = 1974 |isbn = 978-0-309-02217-0 |url = https://books.google.com/books?id=ZZsrAAAAYAAJ|page = 155}}</ref> Chromium was used for electroplating as early as 1848, but this use only became widespread with the development of an improved process in 1924.<ref name="Crplating">{{cite book|title = Nickel and Chromium Plating| publisher = Woodhead Publishing|year = 1993| isbn = 978-1-85573-081-6| pages = 9–12|chapter = History of Chromium Plating|author1=Dennis, J. K. |author2=Such, T. E. }}</ref>

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.<ref name="Hoyt1921">{{cite book | last1 = Hoyt | first1 = Samuel Leslie | title = Metallography, Volume 2 | publisher = McGraw-Hill | year = 1921 }}</ref><ref name="Krupp1888">{{cite book | last1 = Krupp | first1 = Alfred | last2 = Wildberger | first2 = Andreas | title = 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 | publisher = H.C. Baird & Co. | year = 1888 | page = 60 }}</ref><ref name='Gupta1992'>{{cite book | last1 = Gupta | first1 = C.K. | title = Extractive Metallurgy of Molybdenum | publisher = CRC Press | year = 1992 | isbn = 978-0-8493-4758-0}}</ref> 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 molybdenum [[Ductility|ductile]], 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 a [[froth flotation|flotation process]] to recover [[molybdenite]] from ores; flotation remains the primary isolation process. During the [[World War I|first World War]], demand for molybdenum spiked; it was used both in [[Vehicle armor|armor plating]] and as a substitute for tungsten in [[high-speed steel]]s. Some British tanks were protected by 75&nbsp;mm (3&nbsp;in) [[mangalloy|manganese steel]] plating, but this proved to be ineffective. The manganese steel plates were replaced with 25&nbsp;mm (1&nbsp;in) molybdenum-steel plating allowing for higher speed, greater maneuverability, and better protection.<ref name="nbb" /> After the war, demand plummeted until metallurgical advances allowed extensive development of peacetime applications. In [[World War II]], molybdenum again saw strategic importance as a substitute for tungsten in steel alloys.<ref name="Millholland1941">{{cite news | first = Ray | last = Millholland | title = Battle of the Billions: American industry mobilizes machines, materials, and men for a job as big as digging 40 Panama Canals in one year | date = August 1941 | work = Popular Science | page = 61 }}</ref>

In [[World 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 of [[wolframite]] ore at [[Panasqueira]]. Tungsten's resistance to high temperatures and its strengthening of alloys made it an important raw material for the arms industry.<ref name="portugal">{{cite journal|last=Stevens|first=Donald G.|year=1999|title=World War II Economic Warfare: The United States, Britain, and Portuguese Wolfram|journal=The Historian|volume=61|issue=3|pages=539–556|doi=10.1111/j.1540-6563.1999.tb01036.x}}</ref>

==Chemistry==
Unlike other groups, the members of this family do not show patterns in its [[electron configuration]], as two lighter members of the group are exceptions from the [[Aufbau principle]]:
{| class="wikitable"
|-
![[Atomic number|Z]] !! [[Chemical element|Element]] !! [[Electron shell|Numerical Bohr model]]
|-
| 24 || chromium || 2, 8, 13, 1
|-
| 42 || molybdenum || 2, 8, 18, 13, 1
|-
| 74 || tungsten || 2, 8, 18, 32, 12, 2
|-
| 106 || seaborgium || 2, 8, 18, 32, 32, 12, 2
|}

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 the [[periodic table]]. The elements in the group, like those of groups 7–11, have high melting points, and form volatile compounds in higher [[oxidation states]]. All the elements of the group are relatively nonreactive metals with a high melting points (1907&nbsp;°C, 2477&nbsp;°C, 3422&nbsp;°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:<ref name="Schmidt"/> disodium pentacarbonylchromate,  disodium decacarbonyldichromate, [[bis(benzene)chromium]], tripotassium pentanitrocyanochromate, [[chromium(II) chloride]], [[chromium(III) oxide]], [[chromium(IV) chloride]], [[potassium tetraperoxochromate(V)]], and [[chromyl chloride|chromium(VI) dichloride dioxide]]; the same is also true for molybdenum and tungsten, but the stability of the +6 state grows down the group.<ref name="Schmidt">{{cite book|title=Anorganische Chemie II.|chapter = VI. Nebengruppe|pages=119–127|first = Max|last =Schmidt|publisher=Wissenschaftsverlag|year = 1968|language=de}}</ref> Depending on oxidation states, the compounds are basic, amphoteric, or acidic; the acidity grows with the oxidation state of the metal.

==Occurrence and production==

{{expand section|date=February 2012}}

Chromium is a very common [[naturally occurring element]]. It is the [[Abundance of elements in Earth's crust|21st most abundant element]] in the Earth's crust with an average concentration of 100 ppm. The most common [[oxidation state]]s for chromium are zero, trivalent, and hexavalent states. Most naturally occurring chromium is in the [[hexavalent]] state.<ref>{{Cite journal |last=Barnhart |first=J. |date=August 1997 |title=Occurrences, uses, and properties of chromium |journal=Regulatory Toxicology and Pharmacology |volume=26 |issue=1 Pt 2 |pages=S3–7 |doi=10.1006/rtph.1997.1132 |issn=0273-2300 |pmid=9380835}}</ref> About two-fifths of the worlds chromium are produced in [[South Africa]], with [[Kazakhstan]], [[India]], [[Russia]], and [[Turkey]] following. Chromium is mined as chromite ore.

Molybdenum is refined mainly from [[molybdenite]]. It is mainly mined in the United States, China, Chile, and Peru, with the total amount produced being 200,000 tonnes per year.<ref>{{Cite web |title=Molybdenum - Element information, properties and uses {{!}} Periodic Table |url=https://www.rsc.org/periodic-table/element/42/molybdenum |access-date=2023-09-22 |website=www.rsc.org}}</ref>

Tungsten is not a common element on Earth, having an average concentration of 1.5 ppm in Earth's crust. Tungsten is mainly found in the minerals [[wolframite]] and [[scheelite]], and it usually never occurs as a free element in nature. The largest producers of tungsten in the world are China, Russia, and Portugal.

Seaborgium is a [[transuranium element]] that is made artificially by bombarding [[californium]]-249 with [[oxygen]]-18 nuclei. It is artificial, therefore it does not occur in nature.<ref>{{Cite web |title=Seaborgium - Element information, properties and uses {{!}} Periodic Table |url=https://www.rsc.org/periodic-table/element/106/seaborgium |access-date=2023-09-22 |website=www.rsc.org}}</ref>

==Precautions==
[[Hexavalent chromium]] compounds are [[Genotoxicity|genotoxic]] [[carcinogen]]s.<ref>{{cite journal|pmid=22192535|pmc=4138963|year=2012|last1=Wise|first1=SS |title=Chromium and genomic stability |journal=Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis|volume=733|issue=1–2|pages=78–82|last2=Wise|first2=JP Sr|doi=10.1016/j.mrfmmm.2011.12.002|bibcode=2012MRFMM.733...78W }}</ref> Seaborgium is a radioactive synthetic element that is not found in nature; the most stable known isotope has a half-life of approximately 14 minutes.

==Applications==
* Alloys<ref>{{cite web|title=Molybdenum|publisher=AZoM.com Pty. Limited|year=2007|url=http://www.azom.com/details.asp?ArticleID=616|access-date=2007-05-06}}</ref>
* Catalysts
* High temperature and refractory applications, such as welding electrodes and kiln components.
* Metallurgy, sometimes used in jet engines and gas turbines.<ref>{{cite web|title = Nickel-Based Superalloys|first = H. K. D. H.|last = Bhadeshia|url = http://www.msm.cam.ac.uk/phase-trans/2003/Superalloys/superalloys.html|access-date = 2009-02-17|publisher = University of Cambridge|archive-url = https://web.archive.org/web/20060825053006/http://www.msm.cam.ac.uk/phase-trans/2003/Superalloys/superalloys.html|archive-date = 2006-08-25}}</ref>
* Dyes and pigments
* Tanning
* hard materials

==Biological occurrences==
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 in [[enzymes]] of many organisms including humans, and [[tungsten]] has been identified in an analogous role in enzymes from some [[archaea]], such as ''[[Pyrococcus 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 the [[glucose]] metabolism enzyme in some mammals.

==References==
{{Reflist}}

==See also==
{{Periodic table (navbox)}}
{{Navbox periodic table}}
{{Group 6 elements}}

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{{DEFAULTSORT:Group 06}}
[[Category:Groups (periodic table)]]