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Element Fact
Rhenium was the last naturally occurring element to be identified, in 1925. It was so rare that only 1 g of the metal was extracted out of 660 kg of molybdenite ore.

Chromium, Cr

chromium sample

Atomic number: 24
Atomic mass: 51.9961(6)
Natural abundance: 122 ppm
50Cr (49.946050) - 4.345%
52Cr (51.940512) - 83.789%
54Cr (53.938885) - 2.365%

History and Production

Derived from Greek chroma, meaning color. Discovered by L.N. Vauquelin from the mineral crocoite (PbCrO4) in 1797. The metal was isolated in the following year by charcoal reduction. The metal nowadays can be produced by the reduction of chromium(III) oxide (Cr2O3) with aluminium (the Goldschmidt process). Chromium is used to harden steel, in the manufacture of stainless steel and other forms of non-ferrous alloys. It is also used in electroplating to produce decorative and protective surfaces. However, the use of chromium pure metal is limited because of its low ductility.

Physical Data

It is a hard metal, steel-grey in appearance which does not tarnish in air. It consists of 122 ppm of the earth's crust of which the principal ore is chromite (FeCr2O4). The color of the gemstones ruby is due to traces of chromium. In fact, all chromium compounds are beautifully colored, but most of them are toxic and carcinogenic.

Interatomic distance: 249.8 pm
Melting point: 1907C
Boiling point: 2671C
Thermal conductivity/Wm-1K-1: 93.7 (27C)
Density/kgm-3: 7190 (20C), 6460 (m.p.)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 396.6 kJ/mol
Gibbs free energy of formation: 351.8 kJ/mol
Entropy: 174.5 J/mol K
Heat capacity: 20.8 J/mol K

Electronic data

Electronic configuration: [Ar] 3d5 4s1
Term symbol: 7S3
Electron affinity: 64.2593 kJ/mol   Electronegativity (Pauline): 1.66
Ionization energy (first, second, third): 652.868, 1590.63, 2987.19 kJ/mol

Chemical properties

Test for chromium compounds:
(1) Give a green borax bead.
(2) Sulfur dioxide reduces the yellow chromate or orange dichromate to green chromium(III) state (chromic).
(3) Acidified chromate or a dichromate with hydrogen peroxide in the presence of ether gives a blue coloration at the organic layer. This is due to the presence of CrO5, which is not stable in acidic medium. However, With ether, a blue complex of (C2H5)2O.(CrO(O2)2) is formed.

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