Chromium
| General properties | |
|---|---|
| Name, symbol, number | chromium, Cr, 24 |
| Element category | transition metal |
| Group, period, block | 6, 4, d |
| Standard atomic weight | 51.9961(6) |
| Electron configuration | [Ar] 3d5 4s1
2, 8, 13, 1  |
| Discovery | Terracotta Army, China (late 3rd century BC) |
| First isolation | Louis Nicolas Vauquelin (1798) |
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable. The name of the element is derived from the Greek word "chrōma" (χρώμα), meaning colour, because many of its compounds are intensely coloured.
Chromium oxide was used by the Chinese in the Qin dynasty over 2,000 years ago to coat metal weapons found with the Terracotta Army. Chromium was discovered as an element after it came to the attention of the western world in the red crystalline mineral crocoite (lead (II) chromate), discovered in 1761 and initially used as a pigment. Louis Nicolas Vauquelin first isolated chromium metal from this mineral in 1797. Since Vauquelin's first production of metallic chromium, small amounts of native (free) chromium metal have been discovered in rare minerals, but these are not used commercially. Instead, nearly all chromium is commercially extracted from the single commercially viable ore chromite, which is iron chromium oxide (FeCr2O4). Chromite is also now the chief source of chromium for chromium pigments.
Chromium metal and ferrochromium alloy are commercially produced from chromite by silicothermic or aluminothermic reactions, or by roasting and leaching processes. Chromium metal has proven of high value due to its high corrosion resistance and hardness. A major development was the discovery that steel could be made highly resistant to corrosion and discoloration by adding metallic chromium to form stainless steel. This application, along with chrome plating (electroplating with chromium); currently comprise 85% of the commercial use for the element, with applications for chromium compounds forming the remainder.
Trivalent chromium (Cr (III)) ion is possibly required in trace amounts for sugar and lipid metabolism, although the issue remains in debate. In larger amounts and in different forms, chromium can be toxic and carcinogenic. The most prominent example of toxic chromium is hexavalent chromium (Cr (VI)). Abandoned chromium production sites often require environmental cleanup.
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Many of chromium oxide is chromium complexes of chromium compounds and thermal decomposition of the final product, chromate and dichromate in the presence of a reducing agent will get burned chromium oxide.
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