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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] 3d^5 4s^1
                                           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

   CAPTION: 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

                 CAPTION: Main article: Isotopes of chromium

                             iso    NA      half-life   DM DE ( MeV)  DP
                            ^50Cr syn     > 1.8×10^17 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
   Enlarge
   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 (CrO[2]) 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 PbCrO[4], 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 CrO[3],
   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 Cr^3+) 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 (FeCr[2]O[4]) 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,
   Cr[2]O[3], used in enamel painting, and glass staining. Chrome yellow
   is a brilliant yellow pigment, PbCrO[4], used by painters.

   Chromic acid has the hypothetical structure H[2]CrO[4]. Neither chromic
   nor dichromic acid is found in nature, but their anions are found in a
   variety of compounds. Chromium trioxide, CrO[3], 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 \rm
   C_6H_3\mbox{-}2,6(C_6H_3\mbox{-}2,6\mbox{-}Pr^i_2)_2 (Pr^i 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) 1.8x10^17 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.

   Retrieved from " http://en.wikipedia.org/wiki/Chromium"
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   with only minor checks and changes (see www.wikipedia.org for details
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