24
  Cr  
51.996100
Chromium

Name: Chromium
Symbol: Cr
Atomic Number: 24
Atomic Weight: 51.996100
Family: Transition Metals
CAS RN: 7440-47-3
Description: Hard brittle gray transition metal.
State (25 C): Solid
Oxidation states: +2, +3, +6

Molar Volume: 7.78 cm3/mole
Valence Electrons: 3d54s1

Boiling Point:  2945K, 2672C, 4842F
Melting Point:
2130K, 1857C, 3375F
Electrons Energy Level: 2, 8, 13, 1
Isotopes: 22 + 4 Stable
Heat of Vaporization: 344.3 kJ/mol
Heat of Fusion: 16.9 kJ/mol
Density: 7.19 g/cm3 @ 300K
Specific Heat: 0.45 J/gK
Atomic Radius: 1.85
Ionic Radius: 0.52
Electronegativity: 1.66 (Pauling); 1.56 (Allrod Rochow)
Vapor Pressure: 990 Pa @ 1857C

  1s2 2s2p6 3s2p6d5 4s1

History

On July, 26, 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 color in fasion.

In 1797, Louis Nicolas 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. 

PbCrO4 + 2HCl rarrow.gif (63 bytes) CrO+ PbCl2 + H2O

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.

2CrO3  + 3C rarrow.gif (63 bytes) 2Cr + 3CO2

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 indistry and refractory and foundry industries.

Today, chromium is primarily obtained by heating the mineral chromite (FeCr2O4) in the presence of aluminum or silicon.

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

Characteristics

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

1s2
2s2 2p6
3s2 3p6 3d5
4s1

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 passivated by oxygen, forming a thin protective oxide surface layer which prevents oxidation of the underlying metal.

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 dollars.

Though native chromium deposits are rare, some native chromium metal has been discovered.  The Udachnaya Mine in Russia produces 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.

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

Applications

Uses of Chromium:

Compounds

Potassium dichromate is a powerful oxidizing agent and is the preferred compound for cleaning laboratory glassware of any trace organics.  It is used as a saturated solution in concentrated sulfuric 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 oxide (Cr2O3) is the ninth most abundant compound in the earth's crust.

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 anydride of chromic acid, is sold industrially as "chromic acid".

Chrome Green, Chromic Oxide, Chromium (III) Oxide, Cr2O3
Chromium (IV) Oxide, CrO2 Chromium (VI) Trioxide, CrO3
Chromium (III) Chloride, CrCl3 Potassium Dichromate, K2Cr2O7
Chromium (III) Sulfate, Cr2(SO4)3 Chromic Acid, H2CrO4
Mineral Chromite, FeCr2O4 Chromium Boride, CrB
Chromium Hexacarbonyl, Cr(CO)6
Chrome Yellow, Siberian Red Lead, Crocoite, Lead Chromate, PbCrO4

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 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.

Isotopes

Naturally occurring chromium is composed of 4 stable isotopes; 50Cr, 52Cr, 53Cr, and 54Cr with 52Cr being the most abundant (83.789% natural abundance).  22 radioisotopes have been characterized with the most stable being 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 42 amu (42Cr) to 67 amu (67Cr).  The primary decay mode before 52Cr, is electron capture and the primary mode after is beta decay.

atom.gif (700 bytes)

Isotope  
Atomic Mass
 
Half-Life
42Cr 42.00643 14 ms
43Cr 42.99771 21.6 ms
44Cr 43.98555 54 ms
45Cr 44.97964 50 ms
46Cr 45.968359 0.26 seconds
47Cr 46.962900 500 ms
48Cr 47.954032 21.56 hours
49Cr 48.9513357 42.3 minutes
50Cr 49.9460442 Stable
51Cr 50.9447674 27.7025 days
52Cr 51.9405075 Stable
53Cr 52.9406494 Stable
54Cr 53.9388804 Stable
55Cr 54.9408397 3.497 minutes
56Cr 55.9406531 5.94 minutes
57Cr 56.943613 21.1 seconds
58Cr 57.94435 7.0 seconds
59Cr 58.94859 460 ms
60Cr 59.95008 560 ms
61Cr 60.95472 261 ms
62Cr 61.95661 199 ms
63Cr 62.96186 129 ms
64Cr 63.96441 43 ms
65Cr 64.97016 27 ms
66Cr 65.97338 10 ms
67Cr 66.97955 ~10 ms

Precautions

40px-Skull_and_crossbones.svg.jpg (1420 bytes) 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.

Biological Role

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

Recently it was shown, that the popular dietary supplement chromium picolinate complex generates chromosome damage in hamster cells.  In the United States the dietary guidelines for daily chromium uptake were lowered from 50-200 g for an adult to 35 g (adult male) and to 25 g (adult female).

atom.gif (700 bytes)

Chromium Data
 

Atomic Structure

  • Atomic Radius: 1.85
  • Atomic Volume: 7.23cm3/mol
  • Covalent Radius: 1.18
  • Cross Section (Thermal Neutron Capture) Barns: 3.1
  • Crystal Structure: Cubic body centered
  • Electron Configuration:
    1s2 2s2p6 3s2p6d5 4s1
  • Electrons per Energy Level: 2, 8, 13, 1
  • Ionic Radius: 0.52
  • Filling Orbital: 3d5
  • Number of Electrons (with no charge): 24
  • Number of Neutrons (most common/stable nuclide): 28
  • Number of Protons: 24
  • Oxidation States: 6, 3, 2
  • Valence Electrons: 3d5 4s1

Chemical Properties

  • Electrochemical Equivalent: 0.32333 g/amp-hr
  • Electron Work Function: 4.5eV
  • Electronegativity: 1.66 (Pauling); 1.56 (Allrod Rochow)
  • Heat of Fusion: 16.9 kJ/mol
  • Incompatibilities:
    Strong oxidizers (such as hydrogen peroxide), alkalis
  • Ionization Potential
    • First: 6.766
    • Second: 16.5
    • Third: 30.96
  • Valence Electron Potential (-eV): 170

Physical Properties

  • Atomic Mass Average: 51.9961
  • Boiling Point: 2945K, 2672C, 4842F
  • Coefficient of Lineal Thermal Expansion/K-1: 6.2E-6
  • Conductivity
    Electrical: 0.0774 106/cm
    Thermal: 0.937 W/cmK
  • Density: 7.19 g/cm3 @ 300K
  • Description:
    Hard brittle gray transition metal.
  • Elastic Modulus:
    • Bulk: 160.2/GPa
    • Rigidity: 115.3/GPa
    • Youngs: 279/GPa
  • Enthalpy of Atomization: 397.5 kJ/mole @ 25C
  • Enthalpy of Fusion: 15.31 kJ/mole
  • Enthalpy of Vaporization: 348.8 kJ/mole
  • Flammablity Class: Non-combustible solid (except as dust)
  • Freezing Point: see melting point
  • Hardness Scale
    • Brinell: 1120 MN m-2
    • Mohs: 8.5
    • Vickers: 1060 MN m-2
  • Heat of Vaporization: 344.3 kJ/mol
  • Melting Point: 2130K, 1857C, 3375F
  • Molar Volume: 7.78 cm3/mole
  • Physical State (at 20C & 1atm): Solid
  • Specific Heat: 0.45 J/gK
  • Vapor Pressure: 990 Pa @ 1857C

Regulatory / Health

  • CAS Number
    • 7440-47-3
  • RTECS: GB4200000
  • OSHA Permissible Exposure Limit (PEL)
    • TWA: 1 mg/m3
    • Notes: The PEL also applies to insoluble chromium salts.
  • OSHA PEL Vacated 1989
    • TWA: 1 mg/m3
    • Notes: The PEL also applies to insoluble chromium salts.
  • NIOSH Recommended Exposure Limit (REL)
    • TWA: 0.5 mg/m3
    • IDLH: 250 mg/m3
  • Routes of Exposure: Inhalation; Ingestion; Skin and/or eye contact
  • Target Organs: Eyes, skin, respiratory system
  • Levels In Humans:
    Note: this data represents naturally occuring levels of elements in the typical human, it DOES NOT represent recommended daily allowances.
    • Blood/mg dm-3: 0.006-0.11
    • Bone/p.p.m: 0.1-033
    • Liver/p.p.m: 0.02-3.3
    • Muscle/p.p.m: 0.024-0.84
    • Daily Dietary Intake: 0.01-1.2 mg
    • Total Mass In Avg. 70kg human: 14 mg

Who / Where / When / How

  • Discoverer: Nicholas Louis Vauquelin
  • Discovery Location: Paris France
  • Discovery Year: 1797
  • Name Origin:
    Greek: chrma (color).
  • Abundance:
    • Earth's Crust/p.p.m.: 100
    • Seawater/p.p.m.:
      • Atlantic Suface: 0.00018
      • Atlantic Deep: 0.00023
      • Pacific Surface: 0.00015
      • Pacific Deep: 0.00025
    • Atmosphere/p.p.m.: N/A
    • Sun (Relative to H=1E12): 513000
  • Sources:
    Does not occur free in nature. Chromite [Fe,Mg(CrO4)] is its most important mineral. Annual world wide production around 20,000 tons. Primary mining sources are Turkey, South Africa, Zimbabwe, Russia and Philippines.
  • Uses:
    Used to make stainless steel. Also used in plating for car parts, tools, knives, camouflage paint, stereos, video tapes and lasers. It gives rubies and emeralds their color.

Ionization Energy (eV): 6.767 eV
Estimated Crustal Abundance: 1.02102 milligrams per kilogram
Estimated Oceanic Abundance:
310-4 milligrams per liter

Transition Metals
Group 3
(IIIB)
4
(IVB)
5
(VB)
6
(VIB)
7
(VIIB)
8
(VIIIB)
9
(VIIIB)
10 (VIIIB) 11
(IB)
12
(IIB)
Period 4 21
Sc
44.95
22
Ti
47.86
23
V
50.94
24
Cr
51.99
25
Mn
54.93
26
Fe
55.84
27
Co
58.93
28
Ni
58.69
29
Cu
63.54
30
Zn
65.39
Period 5 39
Y
88.90
40
Zr
91.22
41
Nb
92.90
42
Mo
95.94
43
Tc
98.00
44
Ru
101.0
45
Rh
102.9
46
Pd
106.4
47
Ag
107.8
48
Cd
112.4
Period 6 57
La
138.9
72
Hf
178.4
73
Ta
180.9
74
W
183.8
75
Re
186.2
76
Os
190.2
77
Ir
192.2
78
Pt
195.0
79
Au
196.9
80
Hg
200.5
Period 7 89
Ac
227.0
104
Rf
261.0
105
Db
262.0
106
Sg
266.0
107
Bh
264.0
108
Hs
269.0
109
Mt
268.0
110
Ds
269.0
111
Rg
272.0
112
Uub
277.0