CHROMIUM AND ITS CONGENERS

The principal oxidation states of chromium are -1-3 and -1-6. The maximum oxidation number, -f6, corresponds to the position of the element in the periodic table. 

The most important ore of chromium is chromite, FeCr204- The element was not known to the ancients, but was discovered in 1798 in lead chromate, PbCr04, which occurs in nature as the mineral crocoite. 

The metal can be prepared by reducing chromic oxide with metallic aluminum (Section 11-9). Metallic chromium is also made by electrolytic reduction of compounds, usually chromic acid in aqueous solution. 

Although the metal is more electropositive than iron, it easily assumes a passive (unreactive) state, by becoming coated with a thin layer of oxide, which protects it against further chemical attack. This property and its pleasing color are the reasons for its use for plating iron and brass objects, such as plumbing fixtures. 

Chromium in its highest oxidation state (+6) does not form a hydroxide. The corresponding oxide, CrOa, a red substance called chromium(VI) oxide, has acid properties. It dissolves in water to form a red solution of dichromic acid, H2Cr207  

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