|
Cobalt (Co)
| 27 |
iron ← cobalt
→ nickel |
-
↑
Co
↓
Rh |
|
Periodic table - Extended periodic
table | | |
| General |
| Name, symbol, number |
cobalt, Co, 27 |
| Chemical series |
transition metals |
| Group, period, block |
9, 4, d |
| Appearance |
metallic with gray tinge
 |
| Standard atomic weight |
58.933195(5) g·mol−1 |
| Electron configuration |
[Ar] 3d7 4s2 |
| Electrons per shell |
2, 8, 15, 2 |
| Density (near r.t.) |
8.90 g·cm−3 |
| Liquid density at m.p. |
7.75 g·cm−3 |
| Melting point |
1768 K
(1495 °C, 2723 °F) |
| Boiling point |
3200 K
(2927 °C, 5301 °F) |
| Heat of fusion |
16.06 kJ·mol−1 |
| Heat of vaporization |
377 kJ·mol−1 |
| Heat capacity |
(25 °C) 24.81 J·mol−1·K−1 |
Vapor pressure
| P/Pa |
1 |
10 |
100 |
1 k |
10 k |
100 k |
| at T/K |
1790 |
1960 |
2165 |
2423 |
2755 |
3198 | |
| Atomic properties |
| Crystal structure |
hexagonal |
| Oxidation states |
2, 3
(amphoteric oxide) |
| Electronegativity |
1.88 (Pauling scale) |
Ionization energies
(more) |
1st: 760.4 kJ·mol−1 |
| 2nd: 1648 kJ·mol−1 |
| 3rd: 3232 kJ·mol−1 |
| Atomic radius |
135 pm |
| Atomic radius (calc.) |
152 pm |
| Covalent radius |
126 pm |
| Miscellaneous |
| Magnetic ordering |
ferromagnetic |
| Electrical resistivity |
(20 °C) 62.4 nΩ·m |
| Thermal conductivity |
(300 K) 100 W·m−1·K−1 |
| Thermal expansion |
(25 °C) 13.0 µm·m−1·K−1 |
| Speed of sound (thin rod) |
(20 °C) 4720 m/s |
| Young's modulus |
209 GPa |
| Shear modulus |
75 GPa |
| Bulk modulus |
180 GPa |
| Poisson ratio |
0.31 |
| Mohs hardness |
5.0 |
| Vickers hardness |
1043 MPa |
| Brinell hardness |
700 MPa |
| CAS registry number |
7440-48-4 |
| Selected isotopes |
| iso |
NA |
half-life |
DM |
DE (MeV) |
DP |
| 56Co |
syn |
77.27 d |
ε |
4.566 |
56Fe |
| 57Co |
syn |
271.79 d |
ε |
0.836 |
57Fe |
| 58Co |
syn |
70.86 d |
ε |
2.307 |
58Fe |
| 59Co |
100% |
Co is stable with 32 neutrons |
| 60Co |
syn |
5.2714 years |
β- |
2.824 |
60Ni | |
|
|
|
Cobalt is a hard,
lustrous, silver-grey metal, a chemical element with symbol Co. It is found in
various ores, and is used in the preparation of magnetic, wear-resistant, and
high-strength alloys. Its compounds are used in the production of inks, paints,
and varnishes.
Characteristics
Cobalt metal is a silver or gray ferromagnetic. Pure cobalt is not found in
nature, but compounds of cobalt occur naturally in many forms. Small amounts of
it are found in most rocks, soil, water, plants, and animals. It is an element
of atomic number 27. The Curie temperature is of 1388 K with 1.6~1.7 Bohr
magnetons per atom. In nature, it is frequently associated with nickel, and both
are characteristic ingredients of meteoric iron. Mammals require small amounts
of cobalt which is the basis of vitamin B12.
Cobalt-60, an artificially produced radioactive isotope of cobalt, is an
important radioactive tracer and cancer-treatment agent. Cobalt has a relative
permeability two thirds that of iron. Metallic cobalt commonly presents a
mixture of two crystallographic structures hcp and fcc with a transition
temperature hcp→fcc of 722 K. Cobalt has a hardness of 5.5 on the Mohs scale of
mineral hardness.
Common oxidation states of cobalt include +2 and +3, although compounds with
oxidation state +1 are also well developed.
History
Cobalt compounds have been used for centuries to impart a rich blue color to
glass, glazes, and ceramics. Cobalt has been detected in Egyptian sculpture and
Persian jewelry from the third millennium BC, in the ruins of Pompeii (destroyed
AD 79), and in China dating from the Tang dynasty (AD 618–907) and the Ming
dynasty (AD 1368–1644). Cobalt glass ingots have been recovered from shipwrecks
dating to the time of the Minoans (BC 2700-1450).
Swedish chemist George Brandt (1694–1768) is credited with isolating cobalt
in 1735. He was able to show that cobalt was the source of the blue color in
glass, which previously had been attributed to the bismuth found with
cobalt.
During the 19th century, cobalt blue was produced at the Norwegian
Blaafarveværket (70-80% of world production), led by the Prussian industrialist
Benjamin Wegner.
In 1938, John Livingood and Glenn Seaborg discovered cobalt-60.
The word cobalt is derived from the German kobalt, from
kobold meaning "goblin", a term used for the ore of cobalt by miners. The
first attempts at smelting the cobalt ores to produce cobalt metal failed,
yielding cobalt(II) oxide instead; not only that, but because of cobalt's
curious affinity for arsenic, the primary ores of cobalt always contain arsenic,
and upon smelting the arsenic oxidized into the highly toxic
As4O6, which was breathed in by workers.
Biological
Role
Cobalt in small amounts is essential to many living organisms, including
humans. Having 0.13 to 0.30 mg/kg of cobalt in soils markedly improves the
health of grazing animals. Cobalt is a central component of the vitamin
cobalamin, or vitamin B12.
Occurence
Cobalt is not found as a native metal but generally found in the form of
ores. Cobalt is usually not mined alone, and tends to be produced as a
by-product of nickel and copper mining activities. The main ores of cobalt are
cobaltite, erythrite, glaucodot, and skutterudite.
In 2005, the Democratic Republic of the Congo was the top producer of cobalt
with almost 40% world share followed by Canada, Zambia, Russia, Brazil and Cuba,
reports the British Geological Survey.
Applications
Alloys, such as
- Superalloys, for parts in gas turbine aircraft engines.
- Corrosion- and wear-resistant alloys.
- High speed steels.
- Cemented carbides (also called hard metals) and diamond tools.
Magnets and magnetic recording media.
- Alnico magnets.
- Samarium-cobalt magnets.
Catalysts for the petroleum and chemical industries, e.g. for
hydroformylation and oxidation.
Electroplating because of its appearance, hardness, and resistance to
oxidation.
Drying agents for paints, varnishes, and inks.
Ground coats for porcelain enamels.
Pigments (cobalt blue and cobalt green).
Lithium ion battery electrodes.
Steel-belted radial tires.
Purification of histidine-tagged fusion proteins
in biotechnology applications.
Compounds
There is a wide variety of cobalt compounds. The +2 and +3 oxidation states
are most prevalent, however cobalt(I) complexes are also fairly common.
Cobalt(II) salts form the red-pink [Co(OH2)6]2+
complex in aqueous solution. Adding excess chloride will also change the colour
from pink to blue, due to the formation of [CoCl4]2-.
Cobalt oxides are antiferromagnetic at low temperature: CoO (Neel temperature
291 K) and Co3O4 (Neel temperature: 40 K), which is
analogous to magnetite (Fe3O4), with a mixture of +2 and
+3 oxidation states. The oxide Co2O3 is probably unstable;
it has never been synthesized. Other than Co3O4 and the
brown fluoride CoF3 (which is instantly hydrolyzed in water), all
compounds containing cobalt in the +3 oxidation state are stabilized by complex
ion formation.
Isotopes
Naturally occurring cobalt is "monoisotopic", i.e. only one isotope is
stable: 59Co. 22 radioisotopes have been characterized with the most
stable being 60Co with a half-life of 5.2714 years, 57Co
with a half-life of 271.79 days, 56Co with a half-life of 77.27 days,
and 58Co with a half-life of 70.86 days. All of the remaining
radioactive isotopes have half-lives that are less than 18 hours and the
majority of these have half-lives that are less than 1 second. This element also
has 4 meta states, all of which have half-lives less than 15 minutes.
The isotopes of cobalt range in atomic weight from 50 u (50Co) to
73 u (73Co). The primary decay mode for isotopes with atomic mass
unit values less than that of the most abundant stable isotope, 59Co,
is electron capture and the primary mode of decay for those of greater than 59
atomic mass units is beta decay. The primary decay products before
59Co are element 26 (iron) isotopes and the primary products after
are element 28 (nickel) isotopes.
Cobalt radioisotopes in medicine
Cobalt-60 (Co-60 or 60Co) is a radioactive metal that is used in
radiotherapy. It produces two gamma rays with energies of 1.17 MeV and 1.33 MeV.
The 60Co source is about 2 cm in diameter and as a result produces a
geometric penumbra, making the edge of the radiation field fuzzy. The metal has
the unfortunate habit of producing a fine dust, causing problems with radiation
protection. The 60Co source is useful for about 5 years but even
after this point is still very radioactive, and so cobalt machines have fallen
from favor in the Western world where linacs are common.
Cobalt-57 (Co-57 or 57Co) is a radioactive metal that is used in
medical tests; it is used as a radiolabel for vitamin
B12 uptake. It is useful for the Schilling's test.
Industrial uses for radioactive isotopes
Cobalt-60 (Co-60 or 60Co) is useful as a gamma ray source because
it can be produced—in predictable quantity, and high activity—by simply exposing
natural cobalt to neutrons in a reactor for a given time. It is used for
- sterilization of medical supplies, and medical waste;
- radiation treatment of foods for sterilization (cold pasteurization);
- industrial radiography (e.g., weld integrity radiographs);
- density measurements (e.g., concrete density measurements); and
- tank fill height switches.
Cobalt-59 is used as a source in Mössbauer spectroscopy.kl
Precautions
Powdered cobalt in metal form is a fire hazard.
Cobalt compounds should be handled with care due to cobalt's slight
toxicity.
60Co is a high-energy gamma ray emitter. Acute high-dose exposures
to the gamma emissions, such as can occur when irradiation equipment is
inadvertently diverted into scrap, can cause severe burns and death. Extended
exposures increase the risk of morbidity or mortality from cancer.
Nuclear weapon designs could intentionally incorporate 59Co, some
of which would be activated in a nuclear explosion to produce 60Co.
The 60Co, dispersed as nuclear fallout, creates what is sometimes
called a dirty bomb or cobalt bomb, once predicted by physicist Leó Szilárd as
being capable of wiping out all life on
earth.
By: Zookeeper - 2007-12-08 01:14:55
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