Chromium: the essentials
Chromium is steel-gray, lustrous, hard, metallic, and takes a high polish. Its compounds are toxic. It is found as chromite ore. Siberian red lead (crocoite, PrCrO4) is a chromium ore prized as a red pigment for oil paints.
Emerald is a form of beryl (a beryllium aluminium silicate) which is green because of the inclusion of a little chromium into the beryl crytal lattice in place of some of the aluminium ions. Similarly, traces of chromium incorporated into the crystal lattice of corundum (crystalline aluminium oxide, Al2O3) as a replacement for some of the Al3+ ions results in another highly coloured gem stone, in this case the red ruby.
This sample is from The Elements Collection, an attractive and safely packaged collection of the 92 naturally occurring elements that is available for sale.
Cartoon by Nick D Kim ([Science and Ink], used by permission).
Chromium: historical information
In the mid 18th century analysis of Siberian "red lead" (PbCrO4, crocoite) from Siberia showed that it contained quite a lot of lead, but also a further material. This was eventually identified as chromium oxide. Chromium oxide was discovered in 1797 by Louis-Nicholas Vauquelin, who prepared the metal itself in the following year. Starting from crocoite the procedure was to powder the mineral and to precipitate the lead out through its reaction with hydrochloric acid (HCl in water). The residue was chromium oxide, CrO3. Heating this oxide in an oven in the presence of charcoal as a reducing agent gave the metal itself.
Vauquelin also analysed an emerald from Peru and discovered that its green colour is because of the presence of the new element, chromium. In fact, the name chromium is from the Greek word "chroma" meaning "colour", so named because of the many different coloured compounds displayed by chromium.
A year or two after Vauquelin's discovery, a German chemist named Tassaert working in Paris found chromium in an ore now called chromite. This ore, Fe(CrO2)2, is now an important source of chromium.
Chromium around us Read more »
Chromium is an essential trace element and has a role in glucose metabolism. It seems to have an effect in the action of insulin. In anything other than trace amounts, chromium compounds should be regarded as highly toxic.
Chromium is not found as the free metal in nature. The most important ore is chromite (FeCr2O4) and this is found in Turkey, USA, South Africa, Albania, Finland, Iran, Madagascar, Russia, Southern Rhodesia, Transvaal, Cuba, Brazil, Japan, India, Pakistan, and the Philippines.
Crocoite, PbCrO4, is also a chromium mineral and this is found in Russia, Brazil, USA, and Tasmania.
|Location||ppb by weight||ppb by atoms||Links|
|Human||30 ppb by weight||4 atoms relative to C = 1000000|
Physical properties Read more »
Heat properties Read more »
- Melting point: 2180 [1907 °C (3465 °F)] K
- Boiling point: 2944 [2671 °C (4840 °F)] K
- Enthalpy of fusion: |203| kJ mol-1
Crystal structure Read more »
The solid state structure of chromium is: bcc (body-centred cubic).
Chromium: orbital properties Read more »
Chromium atoms have 24 electrons and the shell structure is 18.104.22.168. The ground state electronic configuration of neutral Chromium is [Ar].3d5.4s1 and the term symbol of Chromium is 7S3.
- Pauling electronegativity: 1.66 (Pauling units)
- First ionisation energy: 652.9 kJ mol‑1
- Second ionisation energy: 1590.6 kJ mol‑1
Isolation: it is not normally necessary to make chromium in the laboratory as it is so readily available commercially. The most useful source of chromium commercially is the ore chromite, FeCr2O4. Oxidation of this ore by air in molten alkali gives sodium chromate, Na2CrO4 in which the chromium is in the +6 oxidation state. This is converted to the Cr(III) oxide Cr2O3 by extraction into water, precipitation, and reduction with carbon. The oxide is then further reduced with aluminium or silicon to form chromium metal.
Cr2O3 + 2Al → 2Cr + Al2O3
2Cr2O3 + 3Si → 4Cr + 3SiO2
Another kind of isolation is by electroplating processes. This involves the dissolution of Cr2O3 in sulphuric acid to give an electrolyte used for chromium electroplating.
Chromium isotopes Read more »
Several Chromium isotopes are used for medical applications. Cr-50 is used for the production of the radioisotope Cr-51 which is used for measuring blood volume and red blood cell survival. Cr-53 and Cr-54 are used for the study of chromium metabolism and studies into (adult) diabetes.
|50Cr||49.9460464 (17)||4.345 (13)||0|
|52Cr||51.9405098 (17)||83.789 (18)||0|
|53Cr||52.9406513 (17)||9.501 (17)||3/2||-0.47454|
|54Cr||53.9388825 (17)||2.365 (7)||0|