The chemical element chromium is classed as a transition metal. It was discovered in 1780 by Nicolas Louis Vauquelin.
|Classification:||Chromium is a transition metal|
|Melting point:||1907 oC, 2180 K|
|Boiling point:||2670 oC, 2943 K|
|Neutrons in most abundant isotope:||28|
|Electron configuration:||[Ar] 3d5 4s1|
|Density @ 20oC:||7.19 g/cm3|
Compounds, Radii, Conductivities
|Atomic volume:||7.23 cm3/mol|
|Structure:||bcc: body-centered cubic|
|Specific heat capacity||0.45 J g-1 K-1|
|Heat of fusion||21.0 kJ mol-1|
|Heat of atomization||397 kJ mol-1|
|Heat of vaporization||339.5 kJ mol-1|
|1st ionization energy||652.8 kJ mol-1|
|2nd ionization energy||1592 kJ mol-1|
|3rd ionization energy||2987.2 kJ mol-1|
|Electron affinity||64.3 kJ mol-1|
|Minimum oxidation number||-2|
|Min. common oxidation no.||0|
|Maximum oxidation number||6|
|Max. common oxidation no.||3|
|Electronegativity (Pauling Scale)||1.66|
|Polarizability volume||11.6 Å3|
|Reaction with air||none|
|Reaction with 15 M HNO3||passivated|
|Reaction with 6 M HCl||mild, ⇒ H2, CrCl3|
|Reaction with 6 M NaOH||–|
|Oxide(s)||CrO2, CrO3, Cr2O3 (chromia) + non-stoich|
|Atomic radius||140 pm|
|Ionic radius (1+ ion)||–|
|Ionic radius (2+ ion)||90.5 pm|
|Ionic radius (3+ ion)||75.5 pm|
|Ionic radius (1- ion)||–|
|Ionic radius (2- ion)||–|
|Ionic radius (3- ion)||–|
|Thermal conductivity||93.9 W m-1 K-1|
|Electrical conductivity||7.9 x 106 S m-1|
|Freezing/Melting point:||1907 oC, 2180 K|
Discovery of Chromium
Chromium was discovered in 1780 by French chemist Nicolas Louis Vauquelin in Paris. He discovered the element in a mineral sample of ‘Siberian red lead’- now known as crocoite (lead chromate).
He boiled the crushed mineral with potassium carbonate to produce lead carbonate and a yellow potassium salt solution of chromic acid.
Vauquelin was convinced by further experiments on the solution that he had found a new metal.
In 1781 he succeeded in isolating the metal. Initially he removed the lead from the mineral sample by precipitation with hydrochloric acid. Vauquelin then obtained the oxide by evaporation and finally isolated chromium by heating the oxide in a charcoal oven. (1),(2).
Vauquelin also identified small amounts of chromium in ruby and emerald stones.
Vauquelin went on to discover Beryllium in 1798.
Chromium was named from the Greek word ‘chroma’, meaning color because it forms a variety of colorful compounds.
Appearance and Characteristics
Chromium metal is an essential trace element, but hexavalent chromium Cr(VI) is very toxic and carcinogenic.
Chromium is a silver, lustrous, very hard metal that can take a high mirror polish. It is also odorless, tasteless, and malleable.
The metal forms a thin protective oxide coating in air. It burns when heated to form green chromium oxide Cr2O3.
Uses of Chromium
Chromium is used in stainless steel, and other alloys. Chromium plating, for example on cars and bicycles, produces a smooth, silver finish that is highly resistant to corrosion.
The metal is also widely used as a catalyst.
Chromium compounds are valued as pigments for their vivid green, yellow, red and orange colors.
Abundance and Isotopes
Abundance earth’s crust: 102 parts per million by weight, 40 parts per million by moles
Abundance solar system: 20 parts per million by weight, 0.4 parts per million by moles
Cost, pure: $32 per 100g
Cost, bulk: $0.28 per 100g
Source: Chromium is not found as a free element in nature but is found in the form of ores. The main ore of chromium is chromite FeCr2O4.
To isolate the metal commercially, chromite ore is oxidized to chromium(III) oxide (Cr2O3). The metal is then obtained by heating the oxide in the presence of aluminum or silicon.
Isotopes: Chromium has 21 isotopes whose half-lives are known, with mass numbers 42 to 63. Naturally occurring chromium is a mixture of four isotopes and they are found in the percentages shown: 50Cr (4.3%), 52Cr (83.8%), 53Cr (9.5%) and 54Cr (2.4%).
- Mary Elvira Weeks, Discovery of the Elements V., Journal of Chemical Education, March 1932 p 470.
- Per Enghag, Encyclopedia of the elements: technical data, history, processing, applications.,p 577- 578, John Wiley and Sons, 2004
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