96 | The History and Use of Our Earth’s Chemical Elements Properties
96 | The History and Use of Our Earth’s Chemical Elements Properties
Chromium is a silvery white/gray, hard, brittle noncorrosive metal that has chemical and physical properties similar to the two preceding elements in period 4 (V and Ti). As one of the transition elements, its uses its M shell rather than its outer N shell for valence electrons when combining with other elements. Its melting point is 1,857°C, its boiling point is 2,672°C,
and its density is 7.19 g/cm 3 .
Characteristics Chromium is a hard, brittle metal that, with difficulty, can be forged, rolled, and drawn,
unless it is in a very pure form, in which case the chromium is easier to work with. It is an excellent alloying metal with iron. Its bright, silvery property makes it an appropriate metal to provide a reflective, non-corrosive attractive finish for electroplating.
Various compounds of chromium exhibit vivid colors, such as red, chrome green, and chromate yellow, all used as pigments.
AbundanceandSource Chromium is the 21st most common element found in the Earth’s crust, and chromium
oxide (Cr 2 O 3 ) is the 10th most abundant of the oxide compounds found on Earth. It is not found in a free metallic state. The first source of chromium was found in the mineral crocoite. Today it is obtained from the mineral chromite (FeCr 2 O 4 ), which is found in Cuba, Zimbabwe, South Africa, Turkey, Russia, and the Philippines. Chromite is an ordinary blackish substance that was ignored for many years. There are different grades and forms of chromium ores and compounds, based on the classification of use of the element. Most oxides of chromium are found mixed with other metals, such as iron, magnesium, or aluminum.
Astronauts found that the moon’s basalt rocks contain several times more chromium than is found in basalt rocks of Earth.
History Historically, chromium ore was known as Siberian red lead, which was used to make bright
red paints. The source was soon identified as the mineral crocoite, and analysis indicated that it also contained lead. In 1797 the French chemist Louis-Nicolas Vauquelin (1763–1829) dis- covered chromium while studying some minerals that were collected in Siberia. To isolate the pure metal from its oxide, he first dissolved the lead out of the mineral with hydrochloric acid (HCl), leaving crystals of chromium oxide, which he then heated. To his surprise he ended up with crystals of pure chromium metal.
CommonUses The best-known use of chromium is for the plating of metal and plastic parts to produce
a shiny, reflective finish on automobile trim, household appliances, and other items where a bright finish is considered attractive. It also protects iron and steel from corrosion. It is used to make alloys, especially stainless steel for cookware, and items for which strength and protection from rusting and high heat are important.
Guide to the Elements | 97 Its compounds are used for high-temperature electrical equipment, for tanning leather, as
a mordant (fixes the dyes in textiles so that they will not run), and as an antichalking agent for paints. Some research has shown that, even though most chromium compounds are toxic, a small trace of chromium is important for a healthy diet for humans. A deficiency produces diabetes- like symptoms, which can be treated with a diet of whole-grain cereal, liver, and brewer’s yeast.
Chromium’s most important radioisotope is chromium-51, which has a half-life of about
27 days. It is used as a radioisotope tracer to check the rate of blood flowing in constricted arteries. Some chromium compounds (e.g., chromium chloride, chromic hydroxide, chromic phos- phate) are used as catalysts for organic chemical reactions. In 1960 the first ruby laser was made from a ruby crystal of aluminum oxide (Al 2 O 3 ). These crystals contain only a small amount of chromium, which stores the energy and is responsible for the laser action. A small amount of chromium found in the mineral corundum is respon- sible for the bright red color of the ruby gemstone.
ExamplesofCompounds There are many useful compounds of chromium. As with its preceding two partners in
period four, chromium’s varied oxidation states are responsible for different forms of oxide compounds. For instance, there are the following compounds:
Chromium(II) oxide: Cr 2+ +O 2- → CrO. Used in plating metals.
Chromic(III) oxide: 2Cr 3+ + 3O 2- → Cr 2 O 3 . Green paint pigment. Chromium (IV) oxide: Cr 6+ + 3O 2- → CrO 3. Solution = chromic acid.
In a similar fashion, chromium can form compounds with the halides:
Chromium(II) chloride: Cr 2+ + 2Cl 1- → CrCl 2 . Chromium plating. Chromium(III) fluoride: Cr +3 + 3F 1- → CrF 3 . High-temperature alloys.
Other compounds:
Lead chromate (PbCrO 4 ): known as a chrome yellow pigment. Chromium carbide (Cr 3 C 2 ): resists oxidation at high temperatures. Chromium nitrate [Cr(NO 3 ) 3 ]: corrosion inhibitor, very explosive.
Hazards Most of the compounds of chromium are hazardous when inhaled and irritating when in