Cobalt
2007 Schools Wikipedia Selection. Related subjects: Chemical elements
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| Name, Symbol, Number | cobalt, Co, 27 | ||||||||||||||||||||||||||||||||||||
| Chemical series | transition metals | ||||||||||||||||||||||||||||||||||||
| Group, Period, Block | 9, 4, d | ||||||||||||||||||||||||||||||||||||
| Appearance | metallic with gray tinge |
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| Atomic mass | 58.933195 (5) g/mol | ||||||||||||||||||||||||||||||||||||
| Electron configuration | [Ar] 3d7 4s2 | ||||||||||||||||||||||||||||||||||||
| Electrons per shell | 2, 8, 15, 2 | ||||||||||||||||||||||||||||||||||||
| Physical properties | |||||||||||||||||||||||||||||||||||||
| 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) |
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| Boiling point | 3200 K (2927 ° C, 5301 ° F) |
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| 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 | ||||||||||||||||||||||||||||||||||||
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| Atomic properties | |||||||||||||||||||||||||||||||||||||
| Crystal structure | hexagonal | ||||||||||||||||||||||||||||||||||||
| Oxidation states | 2, 3 ( amphoteric oxide) |
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| 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 | |||||||||||||||||||||||||||||||||||||
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Cobalt ( IPA: /ˈkəʊbɒlt/) is a hard, lustrous, silver-gray 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.
Notable characteristics
Cobalt is ferromagnetic with atomic number 27. The Curie temperature is of 1388 K with 1.6~1.7 Bohr magnetons per atom. It is frequently associated with nickel, and both are characteristic ingredients of meteoric iron. Mammals require small amounts of cobalt salts. 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.
Common oxidation states of cobalt include +2 and +3, though +1 is also seen.
Applications
- Alloys, such as:
- Magnets and magnetic recording media.
- Alnico magnets.
- Catalysts for the petroleum and chemical industries.
- 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).
- Battery electrodes.
- Steel-belted radial tires.
- Cobalt-60 has multiple uses as a gamma ray source:
- It is used in radiotherapy.
- It is used in radiation treatment of foods for sterilization (cold pasteurization).
- It is used in industrial radiography to detect structural flaws in metal parts.
Co-60 is useful as a gamma ray source partially because it can be produced - in known quantity, and very large amounts - by simply exposing natural cobalt to neutrons in a reactor for a given time.
Use 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 favour in the Western world where linacs are common.
History
Cobalt compounds have been used for centuries to impart a rich blue colour 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).
Swedish chemist Georg Brandt (1694–1768) is credited with isolating cobalt sometime between 1730 and 1737. He was able to show that cobalt was the source of the blue colour 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, who thought it worthless and who found that it was poisonous and that it polluted and degraded other mined elements, mainly due to the arsenic and sulfur also found in the ore.
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 B-12.
Occurrence
Cobalt is not found as a free metal and is 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. The world's major producers of cobalt are the Democratic Republic of the Congo, the People's Republic of China, Zambia, Russia and Australia. It is also found in Finland, Azerbaijan, and Kazakhstan. It is also produced in the town of Cobalt, Ontario as a byproduct of silver mining.
- see also Category:Cobalt minerals
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).
- see also Category:Cobalt compounds
Isotopes
Naturally occurring cobalt is composed of 1 stable isotope, 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-lifes 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 amu (50Co) to 73 amu (73Co). The primary decay mode before the most abundant stable isotope, 59Co, is electron capture and the primary mode after is beta decay. The primary decay products before 59Co are element 26 (iron) isotopes and the primary products after are element 28 (nickel) isotopes.
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 powerful gamma ray emitter and exposure to it is therefore a cancer risk. Ingestion of 60Co will lead to incorporation of some cobalt into tissues, which is released very slowly. 60Co is a risk factor in a nuclear confrontation because neutron emissions will convert 59Co into this isotope. Some nuclear weapon designs could intentionally increase the amount of 60Co dispersed as nuclear fallout – this is sometimes called a dirty bomb or cobalt bomb, once predicted by a leading scientist as being capable of wiping out all life on earth. The risk in the absence of a nuclear war comes from improper handling (or theft) of medical radiotherapeutic units. Nevertheless, the gamma radiation emitted from 60Co is used to kill bacteria on fruit and vegetables thus increasing their shelf life. This process is referred to as irradiation.