CHROMIUM This photo shows four different chromium solutions. In the back is an orange solution of potassium dichromate, K2Cr2O7. Next, is a yellow solution of sodium chromate, Na2CrO4. The green solution is chromium(III)chloride, CrCl3. Finally, the violet solution is chromium(III)nitrate, Cr(NO3)3.

Atomic Number: 24
Atomic Weight: 51.996
Electron Configuration: 1s²2s²2p⁶3s²3p⁶4s¹3d⁵
Crystal Structure: Body-Centered Cubic
Atomic Radius: 117 pm
Ionic Radius: 84 pm (Cr²⁺), 69 pm (Cr³⁺)
Density: 7.3 g/cm³
Melting Point: 1857^oC
Important Oxidation States: +2, +3, +6

The name for chromium comes from the Greek word chroma, meaning color; chromium compounds adpopt a wide range of colors and many have been used as pigments. In elemental form,, chromium is hard metal that takes a high polish. The principal ore of chromium is chromite, FeCr₂O₄, from which it is isolated in a complex series of smelting operations. Most people are familiar with chromium due to its widespread use in electroplating; everything from automobile parts to tools is often chromium plated. This accounts for one of the major uses of chromium metal. A second major use is in the manufacture of stainless steel, which contains a minimum of 10.5 - 11.0 percent chromium by mass. Stainless steel is valued for its ability to resist the corrosion common to ordinary steel. This occurs because the chromium forms a thin layer of chromium oxide on the surface of the metal, which protects the underlying metal and precents further corrision.

The +3 oxidation state is one of two stable oxidation states of chromium. Some common chromium compounmds include chromium chloride trihydrate, CrCl₃-3H₂O and potassium chromium sulfate dodecahydrate, KCr(SO₄)₂-12H₂O. The former is green solid consisting of a mixture of [Cr(H₂O)₄Cl₂]+ and [Cr(H₂O)₅Cl]2+ ions. The latter is a midnight-blue soliud.

Chromium(III) is linetically inert, meaning that it does not exchange ligands readily. Therefore, when chromium chloride is dissolved in water, the [Cr(H₂O)₄Cl₂]+ and [Cr(H₂O)₅Cl]2+ ions persist even in aqueous solution. This does not mean that chhromium(III) does not exchange ligands at all, only that it does so very slowly. Such solutions will eventually turn violet over a period of days as [Cr(H₂O)₆]2+ is formed.

Due to the inertness of chromium(III), it has been widely studied and many complexes are known.

The chemistry of chromium is dominated by the chromate (CrO₄^2-) and dichromate (Cr₂O₇^2-) ions. The former is a yellow in color, whereas the latter is orange. Theset two ions exist in equilibrium with one another; adding acid converts chromate into dichromate and adding base converts dichromate back into chromate.

2CrO₄^2- + 2H⁺ = Cr₂O₇^2- + H₂O

These two ions differ greatly in their solubilities. Whereas most dichromates are soluble, several heavy-metal chromates are insoluble. Examples include barium chromate and lead chromate (both of which are the same yellow color as the chromate ion) and silver chromate, which is brick-red in color. Lead chromate, also known as chrome yellow, has been used pigment.

Both the chromate and dichromate ions are strongly oxidizing and are reasily reduced to chromium(III). Care must be taken when these ions are paired with reducing anions such as ammonium. Ammonium dichromate is often used in so-called volcano demonstrations; when ignited is decomposes in a shower of sparks that resembled an erupting volcano.

Chromium(VI) is the highest oxidation state achieve by chromium. Since chromium has only six valence electrons, so the highest oxidiatio state corresponds to the number of valence electrons. This pattern is also followed for titanium (four valence electrons), vanadium (five valence electrons), and manganese (seven valence electrons)