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   Home > Elements > Chromium

vanadium – chromium – manganese
Cr
Mo  
 
 
Image:Cr-TableImage.png
General
Name, Symbol, Number chromium, Cr, 24
Chemical series transition metals
Group, Period, Block 6 (VIB), 4, d
Appearance silvery metallic
Atomic properties
Atomic weight 51.9961 amu
Atomic radius (calc.) 140 (166) pm
Covalent radius 127 pm
van der Waals radius n/a pm
Electron configuration [Ar]3d54s1
e- 's per energy level 2, 8, 13, 1
Oxidation states (Oxide) 6,3,2 (strong acid)
Physical properties
Density 7140 kg/m3
State of matter solid
Melting point 2130 K (3375 °F)
Boiling point 2945 K (4842 °F)
Crystal structure body centered cubic
Hardness 8.5
Magnetic ordering AFM (rather: SDW)
Molar volume 7.23 ×10-6 m3/mol
Heat of vaporization 344.3 kJ/mol
Heat of fusion 16.9 kJ/mol
Vapor pressure 990 Pa at 2130 K
Velocity of sound 5940 m/s at 293.15 K
Miscellaneous
Electronegativity 1.66 (Pauling scale)
Specific heat capacity 450 J/(kg*K)
Electrical conductivity 7.74 106/(m·ohm)
Thermal conductivity 93.7 W/(m*K)
1st ionization potential 652.9 kJ/mol
2nd ionization potential 1590.6 kJ/mol
3rd ionization potential 2987 kJ/mol
4th ionization potential 4743 kJ/mol
5th ionization potential 6702 kJ/mol
6th ionization potential 8744.9 kJ/mol
SI units & STP are used except where noted.

Chromium is a chemical element in the periodic table that has the symbol Cr and atomic number 24.

Notable characteristics

Chromium is a steel-gray, lustrous, hard metal that takes a high polish, is fusible with difficulty, and is resistant to corrosion and tarnishing.

The most common oxidation states of chromium are +2, +3, and +6, with +3 being the most stable. +4 and +5 are relatively rare. Chromium compounds of oxidation state 6 are powerful oxidants.

Applications

Uses of chromium:

  • In metallurgy, to impart corrosion resistance and a shiny finish:
    • as an alloy constituent, e.g. in stainless steel,
    • in chrome plating,
    • in anodized aluminium (literally turning the surface of an aluminium part into ruby).
  • As a catalyst.
  • As a metal polish Cromium 3 Oxide is known as Green rouge.
  • Chromite is used to make molds for the firing of bricks.
  • Chromium salts color glass an emerald green.
  • Chromium salts are used in the tanning of leather.
  • Chromium is what makes a ruby red, and therefore is used in producing synthetic rubies.
  • The chromates and oxides are used in dyes and paints.
  • Potassium dichromate is a chemical reagent, used in cleaning laboratory glassware and as a titrating agent. It is also used as a mordant (i.e. a fixing agent) for dyes in fabric.
  • Chromium Dioxide (CrO2) is used to manufacture magnetic tape, where its higher coercivity than iron oxide tapes gives better performance.

History

In 1761, Johann Gottlob Lehmann found an orange-red mineral in the Ural Mountains which he named Siberian red lead. Though misidentified as a lead compound with selenium and iron components, the material was in fact a lead chromate (PbCrO4).

In 1770, Peter Simon Pallas visited the same site as Lehmann and found a red "lead" mineral that had very useful properties as a pigment in paints. The use of Siberian red lead as a paint pigment developed rapidly. A bright yellow made from crocoite became a very fashionable color.

In 1797, Nicolas-Louis Vauquelin received samples of crocoite ore. He was able to produce chromium oxide (CrO3) by mixing crocoite with hydrochloric acid. In 1798, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven. He was also able to detect traces of chromium in precious gems, such as ruby, or emerald.

During the 1800s chromium was primarily used as a component of paints but now the primary use (85%) is for metal alloys, with the remainder used in the chemical industry and refractory and foundry industries

Chromium was named based on the Greek word "chroma" meaning color, because of the many colorful compounds made from it.

Biological role

Trivalent chromium is an essential trace metal and is required for the proper metabolism of sugar in humans. Chromium deficiencies can affect the potency of insulin in regulating sugar balance. Unlike other essential trace metals, chromium has not been found in a metalloprotein with biological activity. Therefore, the functional basis for the chromium requirement in the diet remains unexplained.

Occurrence

Chromium is mined as chromite (FeCr2O4) ore. Chromium is obtained commercially by heating the ore in the presence of aluminium or silicon. Roughly half the chromite ore in the world is produced in South Africa. Kazakhstan, India and Turkey are also substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa. Approximately 15 million tons of marketable chromite ore were produced in 2000, and converted into approximately 4 million tons of ferro-chrome with an approximate market value of 2.5 billion US dollars.

Though native chromium deposits are rare, some native chromium metal has been discovered. The Udachnaya Mine in Russia produces samples of the native metal. This mine is a kimberlite pipe rich in diamonds, and the reducing environment so provided helped produce both elemental chromium and diamond.

Compounds

Potassium dichromate is a powerful oxidizing agent and is the preferred compound for cleaning laboratory glassware of any possible organics. Chrome green is the green oxide of chromium, Cr2O3, used in enamel painting, and glass staining. Chrome yellow is a brilliant yellow pigment, PbCrO4, used by painters.

Chromic acid has the hypothetical structure H2CrO4. Neither chromic nor dichromic acid is found in nature, but their anions are found in a variety of compounds. Chromium trioxide, CrO3, the acid anhydride of chromic acid, is sold industrially as "chromic acid".

Isotopes

Naturally occurring chromium is composed of 3 stable isotopes; 52-Cr, 53-Cr, and 54-Cr with 52-Cr being the most abundant (83.789% natural abundance). 19 radioisotopes have been characterized with the most stable being 50-Cr with a half-life of (more than) 1.8E17 years, and 51-Cr with a half-life of 27.7025 days. All of the remaining radioactive isotopes have half-lifes that are less than 24 hours and the majority of these have half lifes that are less than 1 minute. This element also has 2 meta states.

Chromium-53 is the radiogenic decay product of 53Mn. Chromium isotopic contents are typically combined with manganese isotopic contents and have found application in isotope geology. Mn-Cr isotope ratios reinforce the evidence from 26Al and 107Pd for the early history of the solar system. Variations in 53Cr/52Cr and Mn/Cr ratios from several meteorites indicate an initial 53Mn/55Mn ratio that suggests Mn-Cr isotope systematics must result from in-situ decay of 53Mn in differentiated planetary bodies. Hence 53Cr provides additional evidence for nucleosynthetic processes immediately before coalescence of the solar system.

The isotopes of chromium range in atomic weight from 43 amu (43-Cr) to 67 amu (67-Cr). The primary decay mode before the most abundant stable isotope, 52-Cr, is electron capture and the primary mode after is beta decay.

Precautions

Chromium metal and chromium(III) compounds are not usually considered health hazards, but chromium (VI) compounds can be toxic if orally ingested. The lethal dose of poisonous chromium (VI) compounds is about one half teaspoon of material. Most chromium (VI) compounds are irritating to eyes, skin and mucous membranes. Chronic exposure to chromium (VI) compounds can cause permanent eye injury, unless properly treated. Chromium(VI) is an established human carcinogen.

In 1958 the World Health Organization recommended a maximum allowable concentration of 0.05 mg/litre in drinking water for chromium (VI), based on health concerns. This recommendation has been reviewed a number of times and this value has not been revised in the meantime.

As chromium compounds were/are used in dyes and paints and the tanning of leather, these compounds are often found in soil and groundwater at (abandoned) industrial sites, now needing environmental cleanup and remediation. See also brownfield land.

References

External links

 

 

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