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Chromium (Cr)

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Chromium single crystal ₂₄Cr^51.996

Discovered and isolated in 1780 by Nicholas Louis Vauquelin at Paris, France.

[Greek: chroma = colour]
  French: chrome
  German: chrom
  Italian: cromo
  Spanish: cromo

Description: Chromium is a hard, blue-white metal. It will dissolve in HCl and H₂SO₄ but not HNO₃, H₂PO or HClO₄ due to reaction and formation of a protective layer on the surface. Chromium can be polished to a high shine and resists oxidation in air. It main uses are in alloys, chrome plating and metal ceramics.

Chromium single crystal properties

State:	Single crystal
Crystal structure:	bcc
Production method:	Floating zone
Standard size:	diameter 6-10mm thickness 1-2mm
Orientation:	(100), (110) and (111)
Orientation accuracy:	<2°, <1°, <0.4° or <0.1d°
Polishing:	as cut, one or two sides polished
Roughness of surface:	<0.03 µm
Purity:	99.999%

Typical analysis (ppm):	Ag < 0.10 C 10.0 Ca 0.01 Cd < 0.10 Cl 1.60 Co < 0.10 Cu < 0.10 Fe 18.0 H < 2.00 Mg < 0.10 Mn < 0.10 Mo < 0.20 N < 5.00 Na < 0.10 Ni 3.00 O < 5.00 Si 4.80 Cr balance

Further Materials properties

Crystal structure:	(cell dimensions/pm), space group, b.c.c. (a=288.46), Im3m
X-ray diffractions mass absorption coefficients:	CuKa 260 (µ/r) / cm2g-1 MoKa 31.1 (µ/r) / cm2g-1
Neutron scattering length:	0.3635 b/10-12 cm
Thermal neutron capture cross-section:	3.1 sa / barns
Density:	7.19 kg/m-3
Melting point:	1856.85 ±20 °C / 2130 ±20 °K
Boiling point:	2671.85 °C / 2945 °K
Molar volume:	7.23 cm3
Thermal conductivity:	93.7 [300 K] Wm-1K-1
Coefficient of linear thermal expansion:	6.2 x 10-6 K-1
Electrical resistivity:	12.7 x 10-8 [293 K] Wm
Mass magnetic susceptibility:	+4.45 x 10-9(s) kg-1m3
Young's modulus:	279 GPa
Rigidity modulus:	115.3 GPa
Bulk modulus:	160.2 GPa
Poisson's ratio:	0.21
Radi:	Cr4+ 56; Cr3+ 64; Cr2+ 84; atomic 1253
Electronegativity:	1.66 (Pauling); 1.56 (Allred); 3.72 eV (absolute)
Effective nuclear charge:	3.45 (Slater); 5.13 (Clementi); 6.92 (Froese-Fischer)
Number of Isotopes (incl. nuclear isomers):	13
Isotope mass range:	45 -> 57

Biological data

Biological role:	Essential to some species, including humans; it is also stimulatory.
Toxicity
Toxic intake:	200 g
Lethal intake:	metal, oral, human = 70 mg kg-1. LD50 (acetate, oral, rat) = 11000 mg kg-1
Hazards:	Chromium is a human poison by ingestion, it is also a suspected carcinogen. Chromates have a corrosive action on skin an tissue
Level in humans
Blood:	0.006 - 0.11 mg dm-3
Bone:	0.1 - 0.33 ppm
Liver:	0.02 - 3.3 ppm
Muscle:	0.024 - 0.84 ppm
Daily dietary intake:	0.01 - 1.2 mg
Total mass of element in average [70 kg] person:	14 mg

Geological data

Minerals:	Many minerals are known, and aluminium is present in many other minerals
Mineral	Formula	Density	Hardness	Crystal apperance
Chromite	FeCr2O4	4.7	5.5	cub., compacted, met. black
Chrocoite	PbCrO4	6.0	2.5 - 3	mon., adam./vit. red-orange

Chief ore:	chromite
World production:	20000 tonnes/year (chromium metal); 9.6 x 106 (chromite ore)
Main mining areas:	Turkey, South Africa, Zimbabwe, Rusia, Philippines.
Reserves:	1 x 109 tonnes
Specimen:	available as chips, chunks, crystallites or powder. Safe.

Abundances
Sun:	5.13 x 105 (relative to H = 1 x 1023)
Earth's crust:	c. 100 ppm
Seawater:
Atlantic surface:	1.8 x 10-4 ppm
Atlantic deep:	2.3 x 10-4 ppm
Pacific surface:	1.5 x 10-4 ppm
Pacific deep:	2.5 x 10-4 ppm
Residence time:	10000 years
Classification:	recycled
Oxidation state:	VI