Chromium

I

Introduction

Chromium, symbol Cr, gray metallic element that can take on a high polish. The atomic number of chromium is 24; the element is one of the transition elements of the periodic table (see Periodic Law).

II

Properties and Occurrence

The element was discovered in 1797 by the French chemist Louis Nicolas Vauquelin, who named it chromium (Greek chroma, “color”) because of the many different colors of its compounds.

Chromium is a common element; overall it ranks about 21st in natural abundance among the elements in crustal rocks. Chromium has an atomic weight of 51.996; the element melts at 1907°C (3465°F), boils at 2672°C (4842°F), and has a specific gravity of 7.2.

Chromium can replace part of the aluminum or iron in many minerals, imparting to them their unique colors. Many precious stones owe their color to the presence of chromium compounds (see Gemstones). Workable ores are rare, however, chromite (FeCr₂O₄) being the only important one.

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In chromites and chromic salts, chromium has a valence of +3. Most of these compounds are green, but some are red or blue. Chromic oxide (Cr₂O₃) is a green solid. In chromates and dichromates, chromium has a valence of +6. Potassium dichromate (K₂Cr₂O₇) is a red, water-soluble solid that, mixed with gelatin, gives a light-sensitive surface useful in photographic processes (see Photography). The chromates are generally yellow, the best known being lead chromate (PbCrO₄), an insoluble solid widely used as a pigment called chrome yellow. Chrome green is a mixture of chrome yellow and Prussian blue.

III

Uses

More than half the production of chromium goes into metallic products, and about another third is used in refractories. It is an ingredient in several important catalysts. The chief use of chromium is to form alloys with iron, nickel, or cobalt. The addition of chromium imparts hardness, strength, and corrosion resistance to the alloy. In the stainless steels, chromium makes up 10 percent or more of the final composition. Because of its hardness, an alloy of chromium, cobalt, and tungsten is used for high-speed metal-cutting tools. When deposited electrolytically, chromium provides a hard, corrosion-resistant, lustrous finish. For this reason it is widely used as body trim on automobiles and other vehicles. The extensive use of chromite as a refractory is based on its high melting point, its moderate thermal expansion, and the stability of its crystalline structure.

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