Monday, February 22, 2010

A Periodic Lent - Chromium

I had intended to post again last week, but things got in the way, so here we are. Today let us consider Chromium, a wonderful metal named for the Greek word for "color". Most people think of chromium from its use as the silver-bright mirror metal in the ornamental parts of cars and motorcycles, but I chose it as the element to suggest the agony in the garden of Gethsemani, since that name means "olive-press": the famous compound chromic oxide, Cr2O3, is used as a green pigment and is rather olive in tone. It "is used as a pigment when chemical and heat resistance are required. It is also used as a ceramic color, for coloring cement, for green granules in asphalt roofing, for camouflage plaints and in the production of chromium metal and Al-Cr master alloys." [The Encyclopedia of Chemical Elements 151. I used this text elsewhere in this posting.]

One of the many fascinating facts about this element is that it was discovered by a Catholic, Louis Nicolas Vauquelin (1763-1829). See here for more about him.

The name "chromium" is aptly chosen, since its compounds can come in many colors. Generally, the chromous salts, Cr++, are blue, whereas the chromic salts, Cr+++, are green or blue or violet. The element also forms anions of great importance: the chromates, CrO4--, are yellow (though the insoluble silver chromate is red); the dichromates, Cr2O7--, are orange-red or red.

The metal itself is hard and blue-white, and crystallizes in the cubic system. It is used in a variety of applications, including coating, plating, and a number of alloys.


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