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Chromium belonging to group 6 and to period 4 of the periodic table belongs to the family of transition metals. It is part of the sixth secondary group of the periodic table, in a restricted sense of the chemical group chromium or group VI B, which also includes molybdenum Mo and tungsten W8.

These elements are characterized by simple metallic bodies which have high melting points, vapor pressures among the lowest, low coefficients of thermal expansion. On the surface of these three metals, an oxide layer (s) is formed at room temperature, often impeding destructive chemical reactions. Stability against weak bases and oxidants is thus ensured. However, molten alkaline media transform these metals into chromate, molybdate and tungstanate ions. The compounds are often colored, sometimes used as mineral pigments. Their different carbides are very hard, their sulfides reveal reticular layered structures, sufficiently stable at the thermal level, hence their applications as lubricants. Alloys with iron have made their reputation for the production of steel tools9. The natural occurrence of the chromium element is fifty times greater than that of tungsten, molybdenum being slightly rarer, of the order of 15% less, than tungsten.

On the element Cr and its discovery
The electronic structure of the chromium atom, ie [Ar] 3d5 4s1 would a priori justify oxidation states between I and VI10

The chemical element as well as the simple body is discovered by Louis-Nicolas Vauquelin from the analysis of red lead from Siberia, or crocoite or chromate from Pb and Cr PbO.CrO3. This mineral had been sent to him by D. Lehmann, mining engineer and director of the collections of natural history in Saint Petersburg, who had as an informed mineralogist described the mineral species in 176611.

The name is explained by the Greek word etymon ֦ὣ̦ or khrôma or latin chroma meaning "color, colors with vivid hues" because the chromium compounds familiar to the chemist are variously colored12. The various shades of its compounds are for example dark carmine for the CrO3 trioxide, green for the Cr2O3 sesquioxide, orange for the sodium dichromate Na2Cr2O7 and other mixed oxides

The uses of chromium metal are varied. Ferrochrome, an intermediate material already presented, is the classic intermediary for the manufacture of special steels, such as stainless steels, Cr and Ni steels, etc.

In metallurgy, chromium makes it possible to make hard and resistant alloys. It is found in anodized aluminum.

Chromium plating or electroplating with chromium improves corrosion resistance and adds a shiny finish to the part. In general, the metal to be treated is previously nickel-plated. The electrolytic deposition of a chromic acid solution in sulfuric acid medium provides a thin protective metallic coating of chromium, a compact layer of around 0.3 m on average.

From September 21, 2017 the use of Chrome (VI) for chromium plating requires an authorization issued by the European Union20.

All of the metallurgical and steel applications, including protection against corrosion, in the late 1990s amounted to around half of the mass uses of Cr. The chromium pigments sector, including pigments for glass and ceramics, represented 25% by mass. The tanning sector, often overlooked, accounted for almost 15% of the chromium consumed, while industrial catalysis techniques required around 5%. Usage in laboratories is obviously tiny.

Chromates and oxides are used as stable pigments in dyes and paints. Yellow chrome, PbCrO4, is a bright yellow pigment used in paint. At the beginning of the 19th century, lead chromate, known for its bright, opaque and light-resistant yellow, was already used as a pigment, the colors it obtained ranged from green yellow to orange yellow, but the product the downside of being toxic.

Certain chromium salts and oxides, of the Cr2O3 type, are used to give a green color to glass and to various vitrified ceramics. "Green chromium" based on chromium oxide Cr2O3 is for example used in paint on enamel.

Chromium alums, etching and tanning agents, are used in the tanning of hides and skins.

Chromium and some of its compounds are catalysts. Thus, in certain hydrogenation reactions, but also in the tricarbonylated form as activating group of a benzene, they allow numerous chemical transformations.

Potassium dichromate is a powerful oxidant used in quantitative chemistry, it is also used in an acid medium for cleaning laboratory glassware in order to remove all organic traces.

In medicine, chromium can be used against diabetes, but its use is controversial.

 

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