Manganese

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25 chromiummanganeseiron
-

Mn

Tc
Mn-TableImage.png
General
Name, Symbol, Number manganese, Mn, 25
Chemical series transition metals
Group, Period, Block 7, 4, d
Appearance silvery metallic
Mn,25.jpg
Atomic mass 54.938045(5) g/mol
Electron configuration [Ar] 3d5 4s2
Electrons per shell 2, 8, 13, 2
Physical properties
Phase solid
Density (near r.t.) 7.21 g/cm³
Liquid density at m.p. 5.95 g/cm³
Melting point 1519 K
(1246 °C, 2275 °F)
Boiling point 2334 K
(2061 °C, 3742 °F)
Heat of fusion 12.91 kJ/mol
Heat of vaporization 221 kJ/mol
Heat capacity (25 °C) 26.32 J/(mol·K)
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k
at T/K 1228 1347 1493 1691 1955 2333
Atomic properties
Crystal structure cubic body centered
Oxidation states 7, 6, 4, 2, 3
(strongly acidic oxide)
Electronegativity 1.55 (Pauling scale)
Ionization energies
(more)
1st: 717.3 kJ/mol
2nd: 1509.0 kJ/mol
3rd: 3248 kJ/mol
Atomic radius 140 pm
Atomic radius (calc.) 161 pm
Covalent radius 139 pm
Miscellaneous
Magnetic ordering nonmagnetic
Electrical resistivity (20 °C) 1.44 µΩ·m
Thermal conductivity (300 K) 7.81 W/(m·K)
Thermal expansion (25 °C) 21.7 µm/(m·K)
Speed of sound (thin rod) (20 °C) 5150 m/s
Speed of sound (thin rod) (r.t.) 198 m/s
Bulk modulus 120 GPa
Mohs hardness 6.0
Brinell hardness 196 MPa
CAS registry number 7439-96-5
Notable isotopes
Main article: Isotopes of manganese
iso NA half-life DM DE (MeV) DP
52Mn syn 5.591 d ε - 52Cr
β+ 0.575 52Cr
γ 0.7, 0.9, 1.4 -
53Mn syn 3.74 ×106 y ε - 53Cr
54Mn syn 312.3 d ε - 54Cr
γ 0.834 -
55Mn 100% Mn is stable with 30 neutrons

Manganese (chemical symbol Mn, atomic number 25) is a gray-white metal that combines with other elements in various proportions. In nature, it occurs mainly in the form of a black-brown oxide (MnO2), which was used as a paint pigment as far back as 17,000 years ago. Manganese is part of the iron group of elements that are thought to be synthesized in large stars shortly before supernova explosion.

Contents

The greatest demand for manganese is for the production of iron and steel. In addition, it is a key component of low-cost stainless steel and certain aluminum alloys. At low concentrations, it is used to decolorize glass, while at higher concentrations, it is used to make violet-colored glass. Manganese dioxide, besides being a useful pigment, is a catalyst and a component of certain dry cell batteries. Potassium permanganate is a potent oxidizer and disinfectant. Manganese (in the form of manganese ions) is an essential trace nutrient in all known forms of life. On the other hand, excess manganese is toxic.

Occurrence

Manganese ore

Manganese occurs principally as the mineral pyrolusite (manganese(IV) oxide, MnO2), and to a lesser extent as rhodochrosite (manganese(II) carbonate, MnCO3). Land-based resources are large but irregularly distributed; those of the United States are very low grade and have potentially high extraction costs. South Africa and Ukraine account for more than 80 percent of the world's identified resources. In addition, manganese is mined in Burkina Faso and Gabon. Vast quantities of manganese exist in manganese nodules on the ocean floor, but attempts to find economically viable methods of harvesting manganese nodules were abandoned in the 1970s.

U.S. import sources (1998-2001):

  • Manganese ore: Gabon, 70 percent; South Africa, ten percent; Australia, nine percent; Mexico, five percent; and other, six percent.
  • Ferromanganese: South Africa, 47 percent; France, 22 percent; Mexico, eight percent; Australia, eight percent; and other, 15 percent.
  • Manganese contained in all manganese imports: South Africa, 31 percent; Gabon, 21 percent; Australia, 13 percent; Mexico, eight percent; and other, 27 percent.

History

Manganese (from the Latin word magnes, meaning "magnet") was in use in prehistoric times, in the form of its compounds. Paints that were pigmented with manganese dioxide (manganese(IV) oxide) can be traced back 17,000 years. The Egyptians and Romans used manganese compounds in glass-making, to either remove color from glass or add color to it. Manganese can be found in the iron ores used by the Spartans. Some speculate that the exceptional hardness of Spartan steels derives from the inadvertent production of an iron-manganese alloy.

In the seventeenth century, German chemist Johann Glauber first produced permanganate, a useful laboratory reagent. (Some, however, believe that it was discovered by Ignites Kaim in 1770). By the mid-eighteenth century, manganese dioxide was in use in the manufacture of chlorine. Swedish chemist Carl Wilhelm Scheele was the first to recognize that manganese was an element, and his colleague, Johan Gottlieb Gahn, isolated the pure element in 1774 by reduction of the dioxide with carbon.

Around the beginning of the nineteenth century, scientists began exploring the use of manganese in steelmaking, and patents were granted for its use at the time. In 1816, it was noted that adding manganese to iron made it harder, without making it any more brittle. In 1837, British academic James Couper noted an association between heavy exposure to manganese in mines with a form of Parkinson's disease. In 1912, patents were issued in the United States for methods of using manganese for "Parkerizing" (electrochemical conversion coating of) firearms to protect them from rust and corrosion.

Notable characteristics

Manganese

In chemistry, manganese is considered a transition metal. It is placed in period four and group seven (former group 7B) of the periodic table. In period four, it lies between chromium (Cr) and iron (Fe), and in group seven, it is positioned above technetium (Tc) and rhenium (Re). Manganese is very brittle, fusible with difficulty, but easily oxidized. After special treatment, manganese metal becomes ferromagnetic—that is, it acquires the "normal" form of magnetism that most people are familiar with.

Manganese combines with various other elements in different proportions. The oxidation states of manganese are known to range from +1 to +7, but the most common ones are +2, +3, +4, +6, and +7. Mn2+ often competes with Mg2+ in biological systems. Compounds in which manganese is in oxidation state +7 are powerful oxidizing agents.

Isotopes

The isotopes of manganese range in atomic weight from 46 atomic mass units (amu) (46Mn) to 65 amu (65Mn). Naturally occurring manganese consists of one stable isotope: 55Mn. In addition, 18 radioisotopes have been characterized, of which the most stable is 53Mn, with a half-life of 3.7 million years. The isotope 54Mn has a half-life of 312.3 days, and 52Mn has a half-life of 5.591 days. The remaining radioactive isotopes have half-lives under three hours, and most of these have half-lives less than one minute.

Manganese is part of the iron group of elements that are thought to be synthesized in large stars shortly before supernova explosion. Given that 53Mn decays to 53Cr, manganese isotopic contents are typically combined with chromium isotopic contents and have found application in isotope geology and radiometric dating. Mn-Cr isotopic ratios reinforce the evidence from isotopic ratios of other elements (26Al and 107Pd) for the early history of the solar system.

Applications

Manganite, a manganese oxide

Manganese is essential to iron and steel production by virtue of its sulfur-fixing, deoxidizing, and alloying properties. Steelmaking, including its ironmaking component, has accounted for most manganese demand—presently in the range of 85–90 percent of the total demand. Among its other uses, manganese is a key component of low-cost stainless steel formulations and certain widely used aluminum alloys. It is also added to gasoline to reduce engine knock. At low concentrations, manganese is used to decolorize glass, as it removes the greenish tinge generated by the presence of iron; at higher concentrations, it is used to make violet-colored glass.

Manganese dioxide, a component of natural umber, is useful as a black-brown pigment in paint. It is also a catalyst and is used in the original type of dry cell battery. Potassium permanganate is a potent oxidizer used in chemical reactions; it is also used in medicine as a disinfectant. The technique known as manganese phosphating (or Parkerizing) is used to prevent the rusting and corrosion of steel. On rare occasions, manganese is used in coins. The only U.S. coins to use manganese were the "wartime" nickel (1942–1945) and the Sacagawea dollar (2000–present).

In 1912, patents were issued in the United States for methods of using manganese for "Parkerizing" (electrochemical conversion coating of) firearms to protect them from rust and corrosion, and these methods have been in widespread use ever since.

Manganese has no satisfactory substitute in its major applications. In minor applications, (such as manganese phosphating), zinc and sometimes vanadium are viable substitutes.

Biological role

Manganese (in the form of manganese ions) is an essential trace nutrient in all known forms of life. Many classes of enzymes have manganese cofactors. They include oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases, lectins, and integrins. The best-known manganese-containing polypeptides (protein-like chains) may be arginase, Mn-containing superoxide dismutase, and the diphtheria toxin.

Compounds

The most stable oxidation state for manganese is +2, and many manganese(II) compounds are known, such as manganese(II) sulfate (MnSO4) and manganese(II) chloride (MnCl2). This oxidation state is also seen in the mineral rhodochrosite, (manganese(II) carbonate). The +3 oxidation state is also known, in compounds such as manganese(III) acetate—these are quite powerful oxidizing agents.

Potassium permanganate (KMnO4), also called Condy's crystals, is an oxidizing agent and a commonly used laboratory reagent. It also finds use as a topical medicine, such as in the treatment of fish diseases.

Manganese(IV) oxide (manganese dioxide, MnO2) is used in dry cells, and can be used to decolorize glass that is polluted by trace amounts of iron. It is also used in the manufacture of oxygen and chlorine and in black paints. Manganese compounds can color glass an amethyst color, and are responsible for the color of true amethyst.

Precautions

Manganese in excess is toxic. Exposure to manganese dusts and fumes should not exceed the upper limit of five milligrams per cubic meter (mg/m3) for even short periods because of its toxicity level.

Acidic permanganate solutions will oxidize any organic material they come into contact with. The oxidation process can generate enough heat to ignite some organic substances.

In 2005, a study suggested a possible link between manganese inhalation and central nervous system toxicity in rats.[1] It is hypothesized that long-term exposure to naturally occurring manganese in shower water puts up to 8.7 million Americans at risk.

A form of Parkinson's disease-type neurodegeneration called "manganism" has been linked to manganese exposure among miners and smelters since the early nineteenth century. Allegations of inhalation-induced manganism have been made regarding the welding industry. In the United States, manganese exposure in workplaces is regulated by the Occupational Safety and Health Administration (OSHA).[2]

See also

Notes

  1. Article Retrieved December 18, 2007.
  2. U.S. Department of Labor Retrieved December 18, 2007.

References

External links

All links retrieved September 14, 2014.

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