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Cobalt

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Cobalt
58.933200(9) +1
+2
+3

  Co
27
1s22s22p63s23p63d74s2 9,4,d
[ ? ] Transition metal:
Properties:
Silver-gray,hard, brittle, lustrous metal
Hazard:
Cobalt dust and fumes are flammable and slightly toxic


Cobalt is a chemical element, having the chemical symbol Co. Its atomic number (the number of protons) is 27. It has a standard atomic weight of 58.9332 g•mol −1, and is a solid in its elemental form.

Cobalt is considered to be a member of the "Transition metal" class of element. At a pressure of 101.325 kPa, it has a boiling point of 1,484 °C, and a melting point of 842 °C. 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.

Contents

Characteristics

Cobalt is ferromagnetic. 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 (Hexagonal Closest Packing) and fcc (Face-Centered Cubic) with a transition temperature hcp → fcc of 449 °C.

Common oxidation states of cobalt include +2, and +3, though +1 is also seen.

Applications

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 favor in the Western world where linacs are common.

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).[1]

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 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, 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.[2]

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, 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 the silver mining.

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).

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, and 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 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.

References

  1. Encyclopedia Britannica Online.
  2. Oxford English Dictionary, Second Edition. 1989.
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