|Name: chromium||Group number: 6|
|Symbol: Cr||Group name: (none)|
|Atomic number: 24||Period number: 4|
|Atomic weight: 51.9961 (6)||Block: d-block|
|CAS Registry ID: 7440-47-3||Voice: |
|Standard state: solid at 298 K||Colour: silvery metallic|
|Classification: Metallic||Availability: |
Nearing Zero cartoon included by kind permission of Nick Kim.
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.
IsolationHere is a brief summary of the isolation of chromium.
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.