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Chromium (chemical symbol Cr) has been categorized previously as one of the trace elements, and has been said to be important for glucose homeostasis, but this is now contentious (see below).
Chromium has the atomic number 24 with an atomic weight of 51.9961 g/mol. Like iron, it is one of the transition metals. It exists in multiple oxidation states, but physiologically the trivalent (Cr3+) state is the most important. In human toxicology hexavalent chromium (Cr6+) is key.
In nature, chromium exists as three stable isotopes: 52Cr, 53Cr and 54Cr. 52Cr is the commonest on Earth, accounting for 83.8% of the total. In total an additional 19 unstable isotopes are known, ranging from the stablest, 50Cr with a half life of 1.8 × 1017 years, and 51Cr with a half-life of 27.7 days, and the rest with half lives all shorter than a day. Several decaying in seconds only.
Since its true biological function remains in the balance, it is difficult to assign a recommended allowance. Nevertheless it is often included in over the counter dietary supplements in its trivalent form and it is not thought to be toxic at these low (microgram) doses.
Its importance in normal human physiology is contentious, indeed the European Union no longer classifies it as an essential trace element, whilst the USA, Japan and Australia still do. It is the only mineral for which the US and European Union do not concur on its essential status.
The toxicity of hexavalent chromium (Cr6+) is widely recognized leading to global bans and restrictions on its use. It is a known carcinogen and also a serious dermatological irritant.
Chromium-containing fumes have also been implicated as a precipitant in metal fume fever, although this is contentious 3.
- chromium-51 is a radioisotope used as a tracer in nuclear medicine for labeling red cells.
History and etymology
In 1761, Johann Gottlob Lehmann (1719–1767) 2, a German geologist, discovered the mineral crocoite in the Urals, which is now known to be lead chromate.
Louis-Nicolas Vauquelin (1763–1829) 2, a French chemist, produced chromium (III) oxide from crocoite in 1797. The following year he extracted impure chromium from reducing chromium (III) oxide with charcoal in 1798.
Chromium originates from the Ancient Greek word, χρώμα (chroma), which means color.