CHEM-I-PAGE

Chromium                                                                                 

Standard Sate: solid  at 298 K

Color: silvery metallic   

Chromium is steel-gray, lustrous, hard, metallic, and takes a high polish. Its compounds are toxic. It is found as chromite ore.

(Cr), chemical element of Group VIb of the periodic table, a hard,
steel-gray metal that takes a high polish and is used in alloys to
increase strength and corrosion resistance. Chromium was
discovered (1797) by the French chemist Nicolas-Louis Vauquelin
and isolated as the metal a year later; it was named for its
multicoloured compounds. The green colour of emerald, serpentine,
and chrome mica and the red colour of ruby are due to chromium.

Occurrence, uses, and properties


Chromium is a relatively abundant element in the Earth's crust. It is
widely dispersed in natural deposits, where it is always combined with
other elements, especially oxygen. Chromite (FeCr2O4) is the only
important commercial mineral.

Chromium is added to iron and nickel in the form of ferrochromium
(about 70 percent chromium) to produce alloys specially
characterized by their high resistance to corrosion and oxidation.
Used in small amounts, chromium hardens steel. Stainless steels are
alloys of chromium and iron in which the chromium content varies
from 10 to 26 percent. Chromium alloys are used to fabricate such
products as oil tubing, automobile trim, and cutlery. Chromite is used
as a refractory and as a raw material for the production of
chromium chemicals.

For additional treatment of chromium metal and its production, see
Industries, Extraction and Processing: Chromium.

Natural chromium consists of a mixture of four stable isotopes:
chromium-52 (83.76 percent), chromium-53 (9.55 percent),
chromium-50 (4.31 percent), and chromium-54 (2.38 percent). The
metal is paramagnetic (weakly attracted to a magnet). It exists in two
forms: body-centred cubic (alpha) and hexagonal close-packed (beta).
At room temperature, chromium slowly dissolves in hydrochloric and
dilute sulfuric acids. Certain oxidizing agents produce a thin oxide
layer on the metal, rendering it passive also to dilute mineral acids,
such as sulfuric. At ordinary temperatures the metal shows no
reaction to seawater or to wet or dry air.


Principal compounds.


The most common oxidation states of chromium are +6, +3, and +2.
A few stable compounds of the +5, +4, and +1 states, however, are
known.

In the hexavalent state, the most important species formed by
chromium are the chromate(VI), CrO42-, and dichromate(VI),
Cr2O72-, ions. These ions form the basis for a series of industrially
important salts. Among them are sodium chromate, NaCrO4, and
sodium dichromate, Na2Cr2O7, which are used in leather tanning, in
metal surface treatment, and as catalysts in various industrial
processes.

Chromium forms several commercially valuable oxygen compounds,
the most important of which is chromium(VI) oxide, commonly called
chromium trioxide or chromic acid, CrO3. An orange-red crystalline
solid, chromatic acid liquefies gradually when exposed to moist air. It
is usually produced by treatment of sodium dichromate with sulfuric
acid. Chromic acid is used chiefly for chromium plating but is also
employed as a colorant in ceramics. It is a powerful oxidant and may
react violently with some organic materials, but such solutions are
often utilized by controlled oxidations in organic synthesis.

Another significant oxygen compound is chromium(III) oxide, also
known as chromium sesquioxide or chromic oxide, Cr2O3. It is
prepared by calcining sodium dichromate in the presence of carbon or
sulfur. Chromium oxide consists of a green powder and is employed
extensively as a pigment; its hydrate form, known as Guignet's green,
is used when chemical and heat resistance are required. For
comparative statistical data on chromium production and reserves,
see mining (table). atomic number 24 atomic weight 51.996 melting
point 1,890 C (3,434 F) boiling point 2,482 C (4,500 F) specific
gravity 7.20 (28 C) valence 2, 3, 6 electronic config. 2-8-13-1 or
(Ar)3d54s1

"chromium" Encyclop�dia Britannica Online.

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