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ChemWiki: The Dynamic Chemistry E-textbook > Inorganic Chemistry > Descriptive Chemistry > d-Block Elements > Group 6: Transition Metals

Group 6: Transition Metals

Chromium (Cr)

Discovered in 1797 by Louis-Nicholas Vauquelin and isolated a year later, chromium is a very hard steel-gray metal which takes its name from the Greek word chroma, for "color", suggesting the wide variety of colors which characterize its compounds, many of which are used as oil paint pigments. the pure metal is also a valuable alloying agent for spring steels and along with nickel, in stainless steels. Chromium(III) oxide (a popular green pigment) is the ninth most abundant chemical in the earth's crust.

Similar in characteristics and practical uses to vanadium and titanium, pure chromium has a blue-white color. It is very brittle and corrosion resistant. It has three common oxidation states: +2, +3, and +6. The metal ore (principally chromite, \(FeCr_2O_4\)), is refined by heating in the presence of Si or Al. The resulting ferrosilicon or ferroaluminum compound is separated from the chromium by electrolysis.

Molybdenum (Mo)

The first significant practical use of this metal (discovered in 1778 by Scheele) was in the alloy used to manufacture the German artillery gun known to the Allies as Big Bertha. The metal takes its name from the Greek word molybdos, meaning "lead" because some of its compounds were once confused with similar lead compounds before Scheele's discovery. The pure metal was first isolated in 1781 by Peter Jacob Hjelm.

Chemically, molybdenum resembles a cross between Cr and W. It has a principal oxidation state of +6 although ionic compounds suggest lower states (e.g., the common ore is \(MoS_2\)).

Molybdenum contributes to the toughness and hardenability of quenched and tempered steels. It is also an essential plant nutrient. Some lands are actually barren for lack of this element in the soil.

Tungsten (W)

The name for the metal tungsten is taken from the Swedish, tungsten, for "heavy stone". The symbol has its origins in the discovery of the element in the late 1700's linked to the mineral wolframite. Most German chemists still refer to the element as Wolfram (wolf's metal). Pure tungsten was eventually isolated in 1783 by brothers Juan and Fausto Elhuyar.

Tungsten is known chiefly as the metal in incandescent light bulb filaments (tungsten has the highest melting point of any metal). But a lot of tungsten is also used in the manufacture of tungsten carbide, an extremely hard material used for making cutting tools and abrasives. The metal is also alloyed with steel for very high temperature applications such as rocket engine nozzles.

Very pure tungsten is actually soft enough to be cut with an ordinary hacksaw but generally small amounts of impurities render the metal very hard and brittle. It i very similar to both Mo and Cr. It is straddled by elements having the second and third highest melting points, rhenium, Re, and tantalum, Ta.

Seaborgium (Sg)

The synthesis of element 106 was first reported by Soviet scientists in June of 1974. Three months later the Berkeley group claimed positive discovery of the same element. The Soviet claim was based on bombardment of three different isotopes of lead with high energy chromium-54 ions. The American group used a californium target and ions of oxygen-18. The longest lived isotope (which has no known practical uses) has a half-life of 800 milliseconds.

In early1997 the first aqueous chemistry of Sg was reported out of Germany. The most stable oxidation state in aqueous solution appears to be +6 and formation of neutral or anionic oxo or oxohalide compounds occurs just as for Mo or W. The early experiments show, as expected, that Sg is chemically like other members of group 6 and not like uranium [this goes back to an early controversy about the so-called transuranium elements: would they exhibit chemical properties similar to uranium and thus need to be removed to a separate section of the periodic table, or would they--in spite of their radioactivity--exhibit expected chemical properties of their groups? Time and research have shown the latter to be correct].

Like elements 104 and beyond, the controversy over the discovery and therefore the naming of 106 led the IUPAC to adopt the numerical symbol Unh for element 106 (unnilhexium). But in 1993 the Berkeley group gave definitive proof of their priority in synthesis and gave the name Seaborgium (Sg) after Glenn T. Seaborg. While the choice of name was lauded in general, the IUPAC was awaiting a list of names from a committee for elements 104-109. The committee rejected Seaborgium because Seaborg is still living. The Berkeley group pointed out that Einsteinium and Fermium were both named after living scientists at the time (although both died before the names were officially accepted). The committee suggested Rutherfordium (Rf) but after many complaints about the choice of names, reverted to Seaborgium.

Seaborgium is the name which the IUPAC finally approved in August 1997.

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Last modified
14:12, 24 Jan 2015

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