Manganese

54.93

Cave paintings, 16 000 years old, in Lascaux, France. The black color was made by our ancestors using manganese dioxide.

Classification:	Manganese is a transition metal
Atomic weight:	54.9380 g/mol
Density @ 20 ^oC:	7.43 g/cm³
Atomic volume:	7.4 cm³/mol

Manganese Ores: Rhodochrosite (manganese carbonate) on left: pink Mn²⁺. Pyrolusite (manganese dioxide) on right: black Mn⁴⁺.

Without manganese, this picture would not exist. In fact, there would be no advanced life forms on our planet. Manganese is essential for photosynthesis.

Manganese dioxide makes the black color in black smokers - hot geothermal vents on the ocean floor. Image courtesy: NASA.

Discovery of Manganese

Author: Dr. Doug Stewart

Manganese compounds have been used since ancient times. The image above (right) shows cave paintings made 16 000 years ago with manganese dioxide.

In the first century A.D., Pliny the Elder, the Roman author, described how black powder (manganese dioxide) was used to manufacture colorless glass. ⁽¹⁾ It is still used for the same purpose today.

The first recognition of manganese's existence as a distinct element was in 1740, when the German chemist, Johann Heinrich Pott, stated that pyrolusite (manganese dioxide) contained a new earth metal. Until then pyrolusite was believed to have been a compound of iron.

Pott prepared potassium manganate by fusing caustic potash (potassium hydroxide) with pyrolusite in air. The color changes he observed in the product were green/blue/red/green, establishing that pyrolusite did not contain iron. ^(2),(2a),(3)

Details of the first isolation of metallic manganese were published in 1770 by Ignatius Gottfried Kaim in his dissertation, under the supervision of the chemist Jakab Jozsef Winterl.

Working in Vienna, Kaim mixed powdered pyrolusite with twice its weight of black flux and heated the mixture strongly. The composition of Kaim's flux is uncertain, although it was probably based on charcoal. (A flux is a reducing agent.) If so, the reaction would have been:

MnO₂ + C → Mn + CO₂

Kaim described the product of the reaction as a blue-whitish shiny brittle metal with many differently shaped facets. When broken and viewed side-on, he observed blue flecks.

Kaim claimed that no iron was present in his product, but he was aware his manganese was not pure. He invited other chemists to find a flux that could produce the pure metal. ^{(4), (5)}

In Sweden, Carl Wilhelm Scheele - the discoverer of chlorine and one of oxygen's independent discoverers - was aware that pyrolusite contained a new element. His attempts to isolate it failed and he asked his friend Johan Gottlieb Gahn to try.

Gahn used a similar method to the one Kaim had used a few years earlier, with charcoal as the reducing agent. Gahn used a blowpipe to increase the temperature of the reaction. The metal he produced was white, hard and brittle; when broken it had a granular structure. Gahn's manganese was also impure, but the new metallic element now gained widespread recognition. ⁽²⁾

Manganese's name comes from the use of its compounds in glass-making. These compounds were called 'magnes' in Latin, meaning 'magnet.' ⁽⁶⁾

Manganese is not actually magnetic!

Interesting Facts: Did You Know?

Interesting Facts about Manganese: Did You Know?

Manganese is an essential element in photosynthesis. Without it, there would be no free oxygen on earth.
Human bodies contain a tiny amount of manganese: about 10 - 20 mg. If you chopped a 10c coin into 100 equal pieces, each piece would weigh more than the weight of manganese in an average human. Despite this, we cannot survive without it; manganese performs vital metabolic functions.
Manganese in humans needs to be topped up frequently because our bodies cannot store it.
Neanderthals may have used black manganese dioxide as a cosmetic 50 000 years ago. ^(7),(8)

States

State (s, l, g): solid
Melting point: 1523 K (1250 ^oC)

Boiling point: 2333 K (2060 ^oC)

Energies

Specific heat capacity: 0.48 J g^-1 K^-1
Heat of fusion: 12.91 kJ mol^-1
1^st ionization energy: 717.4 kJ mol^-1
3^rd ionization energy: 3248.3 kJ mol^-1

Heat of atomization: 283 kJ mol^-1
Heat of vaporization: 219.74 kJ mol^-1
2^nd ionization energy: 1509 kJ mol^-1
Electron affinity: kJ mol^-1

Oxidation & Electrons

Shells: 2,8,13,2
Minimum oxidation number: -3
Min. common oxidation no.: 0
Electronegativity (Pauling Scale): 1.55

Electron configuration: [Ar] 3d⁵ 4s²
Maximum oxidation number: 7
Max. common oxidation no.: 2
Polarizability volume: 9.4 Å³

Appearance & Characteristics

Structure: special: complex (cubic)
Hardness: 6.0 mohs

Transition metals are well-known for their multiple colored ions. Watch the changes in color as manganese changes its oxidation state from +7 to +6 to +4.

Colors on opposite sides of the color wheel cancel.

Ever wondered how drink cans can be so thin? Manganese stiffens aluminum, so less of it is needed to make soft drink cans.

Color: gray-white

Harmful effects:
Excess manganese, particularly inhalation of the powder/dust, is toxic. Exposure to manganese or manganese oxide(s) dust can result in a medical condition called manganism; symptoms resemble those of Parkinson's disease. Workers at particular risk of exposure include miners and welders.

Characteristics:
Manganese is a gray-white, hard, brittle, metal that can take a high polish. It is not magnetic. The metal tarnishes on exposure to air and, when heated, oxidizes to manganese(II, III) oxide (Mn₃O₄).

Like other transition metals, manganese has more than one common oxidation state. The most stable is +2, which is a pale pink color in aqueous solutions. Also important is +4, brown/black, which is found in manganese dioxide; and +7 found in the purple permanganate anion MnO₄^-. Manganese's +6 oxidation state is green.

Uses:
For over 2000 years manganese dioxide has been used to make colorless glass. Glass is made from sand (silica) and most sand contains iron (II) oxide, which naturally gives glass a green color. It is sometimes incorrectly stated that the manganese dioxide oxidizes the iron (II) oxide to iron (III) oxide; with a corresponding color change from intense green to pale yellow - and the pale yellow is too faint to be seen.

In fact, the real reason the glass decolorizes is complementary colors. Colors that are directly opposite one another on the color wheel (image: left) cancel one another out, to leave a pale gray.

In glass the manganese dioxide forms a violet silicate which cancels the green color of Iron (II). ⁽⁹⁾

Manganese dioxide is also used as a black-brown pigment in paint and as a filler in dry cell batteries.

The great majority of manganese ore ends up in steel production where the manganese desulfurizes and deoxidizes the steel.

It is also is used extensively to produce a variety of important alloys. For example, the aluminum used to make most soft drinks cans contains about 1% manganese to improve the cans' stiffness and corrosion resistance.

Organo-manganese compounds can be added to gasoline to increase its octane rating and reduce engine knock.

Manganese is the twelfth most abundant element in the earth's crust and it is an essential trace element for all life on earth.

In the human body several manganese-containing enzymes are need to metabolize carbohydrates, cholesterol, and amino acids. Typically our bodies have about 10 - 20 mg manganese. This needs to be topped up frequently because our bodies cannot store it. About a quarter of the manganese in our bodies is in bone, while the rest is evenly distributed through our tissues. ⁽¹⁰⁾

Reactions & Compounds

Reaction with air: mild, w/ht ⇒ Mn₃O₄, Mn₃N₂
Reaction with 15 M HNO₃: mild, ⇒ Mn(No₃)₂, No_x
Oxide(s): MnO, Mn₃O₄, Mn₂O₃, Mn₂O₇
Hydride(s): None

Reaction with 6 M HCl: mild, ⇒ H₂, MnCl₂
Reaction with 6 M NaOH:
Chloride(s): MnCl₂

Radius

Atomic radius: 140 pm
Ionic radius (2+ ion): 89 pm
Ionic radius (2- ion): pm

Ionic radius (1+ ion): pm
Ionic radius (3+ ion): 75.3 pm
Ionic radius (1- ion): pm

Conductivity

Thermal conductivity: 7.81 W m^-1 K^-1

Electrical conductivity: 0.5 x 10⁶ S m^-1

Abundance & Isotopes

Abundance earth's crust: 0.1% by weight, 360 parts per million by moles
Abundance solar system: 10 parts per million by weight, 0.2 parts per million by moles
Cost, pure: $6.5 100g
Cost, bulk: $0.28 per 100g

Source: Manganese occurs mainly as the mineral pyrolusite (MnO₂) and as rhodochrosite (MnCO₃). Manganese can be found in manganese nodules on the ocean floor, but these are currently commercially unviable. Commercially, manganese is produced by reduction of its ores using aluminum. High purity manganese is made by electrolysis of manganese sulfate in solution.

Isotopes: Manganese has 21 isotopes whose half-lives are known, with mass numbers 46 to 66. Of these, only one is stable, ⁵⁵Mn.

References

1. S. Venetskii, Metal from black stone., Metallurgist, 1 July 1966, Vol 10, Issue 7, p423-425.
2. Mary Elvira Weeks, The discovery of the elements. III. Some eighteenth-century metals., J. Chem. Educ., 1932, 9 (1), p22.
2a. Mary Elvira Weeks, The discovery of the elements., 1960, Sixth Edition, p169-173, Journal of Chemical Education.
3. Leopold Gmelin, Handbuch der Theoretischen Chemie., 1827, Vol. 1, Part 2, p882.
4. Peter Joseph Macquers, Chymisches Wörterbuch oder Allgemeine Begriffe der Chymie nach Alphabetischer Ordnung, 1788, p572-573.
5. J.C. Wiegleb, Geschichte des Wachsthums und der Erfindungen in der Chemie in der neuern Zeit, Zweyter Band von 1750 bis 1790., 1791, p104.
6. Saul S. Hauben, The derivations of the names of the elements, J. Chem. Educ., 1933, 10 (4), p227.
7. Winfried Henke, Thorolf Hardt, Handbook of Paleoanthropology, 2007, Volume 1, p1733, Springer.
8. BBC Report, Neanderthal 'make-up' containers discovered 9 January 2010.
9. David Dingledy, Decolorization of glass., J. Chem. Educ., 1965, 42 (3), p160.
10. J. S. Garrow, W.P.T. James, A Ralph, Human nutrition and dietetics., 2000, p202, Churchill Livingstone.

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