Chromium

2007 Schools Wikipedia Selection. Related subjects: Chemical elements

24 vanadium ← chromium → manganese - ↑ Cr ↓ Mo Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	chromium, Cr, 24
Chemical series	transition metals
Group, Period, Block	6, 4, d
Appearance	silvery metallic
Atomic mass	51.9961 (6) g/mol
Electron configuration	[Ar] 3d5 4s1
Electrons per shell	2, 8, 13, 1
Physical properties
Phase	solid
Density (near r.t.)	7.15 g·cm−3
Liquid density at m.p.	6.3 g·cm−3
Melting point	2180  K (1907 ° C, 3465 ° F)
Boiling point	2944 K (2671 ° C, 4840 ° F)
Heat of fusion	21.0 kJ·mol−1
Heat of vaporization	339.5 kJ·mol−1
Heat capacity	(25 °C) 23.35 J·mol−1·K−1
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 1656 1807 1991 2223 2530 2942
Atomic properties
Crystal structure	cubic body centered
Oxidation states	6, 4, 3, 2 (strongly acidic oxide)
Electronegativity	1.66 (Pauling scale)
Ionization energies ( more)	1st: 652.9 kJ·mol−1
	2nd: 1590.6 kJ·mol−1
	3rd: 2987 kJ·mol−1
Atomic radius	140 pm
Atomic radius (calc.)	166 pm
Covalent radius	127 pm
Miscellaneous
Magnetic ordering	AFM (rather: SDW)
Electrical resistivity	(20 °C) 125 nΩ·m
Thermal conductivity	(300 K) 93.9 W·m−1·K−1
Thermal expansion	(25 °C) 4.9 µm·m−1·K−1
Speed of sound (thin rod)	(20 °C) 5940 m/s
Young's modulus	279 GPa
Shear modulus	115 GPa
Bulk modulus	160 GPa
Poisson ratio	0.21
Mohs hardness	8.5
Vickers hardness	1060 MPa
Brinell hardness	1120 MPa
CAS registry number	7440-47-3
Selected isotopes
Main article: Isotopes of chromium iso NA half-life DM DE ( MeV) DP 50Cr syn > 1.8×1017 y εε - 50Ti 51Cr syn 27.7025 d ε - 51V γ 0.320 - 52Cr 83.789% Cr is stable with 28 neutrons 53Cr 9.501% Cr is stable with 29 neutrons 54Cr 2.365% Cr is stable with 30 neutrons
References

Chromium ( IPA: /ˈkrəʊmiəm/) is a chemical element in the periodic table that has the symbol Cr and atomic number 24.

Notable characteristics

Chromium

Chromium is a steel-gray, lustrous, hard metal that takes a high polish, and has a high melting point. It is also odourless, tasteless, and is somewhat malleable.

The most common oxidation states of chromium are +2, +3, and +6, with +3 being the most stable. +1, +4 and +5 are rare. Chromium compounds of oxidation state 6 are powerful oxidants.

Chromium is passive with oxygen, i.e. forms a very thin, insoluble, protective oxide which prevents further oxidation of the underlying metal.

Applications

Uses of chromium:

• In metallurgy, to impart corrosion resistance and a shiny finish :-
  • as an alloy constituent, such as in stainless steel in cutlery
  • in chrome plating,
  • in anodized aluminium, literally turning the surface of aluminium into ruby.
• As dyes and paints :-
  • Chromium(III) oxide is a metal polish known as green rouge.
  • Chromium salts colour glass an emerald green.
  • Chromium is what makes a ruby red, and therefore is used in producing synthetic rubies.
  • also makes a brilliant yellow for painting
• As a catalyst.
• Chromite is used to make molds for the firing of bricks.
• Chromium salts are used in the tanning of leather.
• 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(IV) oxide (CrO₂) is used to manufacture magnetic tape, where its higher coercivity than iron oxide tapes gives better performance.
• In well drilling muds as an anti-corrosive.
• In medicine, as a dietary supplement or slimming aid, usually as chromium (III) chloride or chromium(III) picolinate.

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 lead chromate with a formula of PbCrO₄, now known as the mineral crocoite.

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 colour in fashion.

In 1797, Nicolas-Louis Vauquelin received samples of crocoite ore. He was able to produce chromium oxide with a chemical formula of 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 gemstones, such as ruby, or emerald.

During the 1800s chromium was primarily used as a component of paints and in tanning salts but now the primary use in is for metal alloys and is responsible for 85% of the use of chromium. The remainder is used in the chemical industry and refractory and foundry industries.

Chromium was named after the Greek word "chroma" meaning colour, because of the many colorful compounds made from it.

Biological role

Trivalent chromium (Cr(III), or Cr³⁺) is required in trace amounts for sugar metabolism in humans, and its deficiency can cause chromium deficiency. In contrast, hexavalent chromium is very toxic.

Occurrence

Chromium is mined as chromite (FeCr₂O₄) ore. 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 United States 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.

Isolation

Chromium is obtained commercially by heating the ore in the presence of aluminium or silicon.

Compounds

Potassium dichromate is a powerful oxidizing agent and is the preferred compound for cleaning laboratory glassware of any possible organics. It is used as a saturated solution in concentrated sulphuric acid for washing the apparatus. For this purpose, however, sodium dichromate is sometimes used because of its higher solubility (5 g/100 ml vs. 20 g/100 ml respectively). 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".

Chromium and the quintuple bond

Chromium is notable for its ability to form quintuple covalent bonds. Writing in Science, Tailuan Nguyen, a graduate student working with Philip Power of the University of California, Davis describes the synthesis of a compound of chromium(I) and a hydrocarbon radical which was shown via X-ray diffraction to contain a quintuple bond of length 183.51(4) pm (1.835 angstroms) joining the two central chromium atoms. This was accomplished through the use of an extremely bulky monodentate ligand which through its sheer size, is able to prevent further coordination. Chromium currently remains the only element for which quintuple bonds have been observed.

The compound had the Lewis structure

Ar − Cr − Cr − Ar

where Ar is the aryl group (Prⁱ is isopropyl)

Isotopes

Naturally occurring chromium is composed of 3 stable isotopes; ⁵²Cr, ⁵³Cr, and ⁵⁴Cr with ⁵²Cr being the most abundant (83.789% natural abundance). 19 radioisotopes have been characterized with the most stable being ⁵⁰Cr with a half-life of (more than) 1.8x10¹⁷ years, and ⁵¹Cr with a half-life of 27.7 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.

⁵³Cr 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 (⁴³Cr) to 67 amu (⁶⁷Cr). The primary decay mode before the most abundant stable isotope, ⁵²Cr, 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 hexavalent chromium (chromium VI) compounds can be toxic if orally ingested or inhaled. 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. An investigation into hexavalent chromium release into drinking water formed the plot of the motion picture Erin Brockovich.

World Health Organization recommended maximum allowable concentration in drinking water for chromium (VI) is 0.05 milligrams per liter.

As chromium compounds were used in dyes and paints and the tanning of leather, these compounds are often found in soil and groundwater at abandoned industrial site, now needing environmental cleanup and remediation per the treatment of brownfield land. Primer paint containing hexavalent chromium is still widely used for aerospace and automobile refinishing applications.