Manganese in Periodic Table
Manganese (Mn), chemical element, a greyish-white hard, brittle paramagnetic metal of Group 7 (VIIB) of the periodic table used mainly as an additive to steel. The Swedish chemist Bergmann discovered the presence of magnesium in the black magnesia but fails to isolate it. In chemistry, the element was isolated by another Swedish chemist Johan Gottlieb Gahn and studies by the Swedish chemist Carl Wilhelm Scheele in 1774. At first, the metal was named in Latin word magnesium from the old name pyrolusite but in 1808, the Latin name of manganese changed to manganism to avoid confusion.
The body-centered cubic crystal lattice, manganese has chemical symbol Mn, atomic number 25, atomic weight 54.938, melting point 1246°C, boiling point 2061°C, and valence shell electron configuration [Ar] 3d5 4s2. The melting point and density of manganese are much lower than those of chromium suggests the more participation of d-electron in metallic chemical bonding. The +6 oxidation number or state is familiar to us in the compound potassium permanganate, KMnO4, a strong oxidizing agent.
Occurrence and Production of Manganese
Manganese is the third most abundant transition metal after iron and titanium and the twelfth most abundant element among all periodic table elements. It occurs 1000 ppm in the earth’s crust and a variety of minerals like pyrolusite (MnO2) Hausmannite (Mn3O4), Braunite (Mn2O3) Manganese spar (MnCO3) in Russia, India, South Africa, Ghana, Brazil, and Chile. Colloidal particles of manganese are found on the ocean floor to form compact Mn nodules, which contain 15 to 30 percent of metal on a dry basis with copper, nickel, and cobalt. More than 1012 tones of such nodules have already been accommodated from the ocean water bed. The large deposit of the elements may gain commercial significance in the future. The transition element, manganese present in food plants and animal bodies and plays an important role in the biological process.
Naturally, occurring manganese has one stable isotope with atomic mass 55 and several radioactive isotopes with atomic mass range 44 to 69. The radioactive isotopes of manganese are obtained from different types of nuclear reaction and 55Mn (half-life = 3.7 million years) and 54Mn (half-life = 312.2 days) are the most stable ones.
The element, manganese is produced by electrolysis of an aqueous solution of Mn(II) sulfate which is prepared from pyrolusite by heating concentrated sulfuric acid at 150°C. Alternatively, pyrolusite heated below 800°C with dehydrated green vitriol. In both techniques, manganese sulfate (MnSO4) is leached out from the residue by leaving impurities like iron oxide and other silicate compounds. The Mn(II) sulfate was further purified by crystallization before subjecting electrolysis.
Chemical Properties and Compounds
The greyish-white paramagnetic metal, manganese does not oxidize in very pure form but the presence of impurities like carbon makes it more reactive. It liberated hydrogen from all dilutes acids, including dilute nitric acid. The acids are reduced to form sulfur dioxide and nitrogen monoxide (NO). The +2 oxidation state is most stable due to the presence of a half-filled d5 electron configuration. The reduction potential oxidation state diagram suggests that Mn(VI) is a highly oxidizing agent, oxidizes to form Mn(III) and Mn(IV) ions.
Chemistry of Mn(VII)
Manganese(VII) compounds like KMnO4 is familiar to us due to their strong oxidizing properties. Permanganate ion is a very strong oxidizing agent in acid solution and moderately strong in the neutral and alkaline medium used as an oxidizer in analytical chemistry. In the +7 state, the metal forms oxide like Mn2O7 and oxohalides MnO3Cl and MnO3F. The green oxide is highly explosive that ignites alcohol or ether when brought into contact.
Chemistry of Mn(VI)
Manganate ion (MnO4-2) is the only suitable representative in the +6 state of manganese, obtained as a green mass on fusing pyrolusite with alkali. The green mass is extracted with water containing little alkali and the solution evaporated to yield dark green crystalline solid K2MnO4.
Chemistry of Mn(IV)
In the +4 state, manganese form stable dioxide MnO2, halide MnF4, and the complexes MnX6-2, where X = fluorine, chlorine, cyanide, and IO3. Pyrolusite or MnO2, a common mineral present in the earth’s crust, is the main source of the metal manganese. It is a slightly brownish-black solid mostly insoluble in water and inert to cold acids. Hydrochloric acid and heated concentrated sulfuric or nitric acid is oxidized by MnO2. Manganese dioxide is an important compound of Mn, widely used glass industry to decolorize glass and in the manufacturing of matches box.
Chemistry of Mn(II)
The +2 state is the most common and stable oxidation state of manganese to form a large number of binary compounds (oxides, halides) and complexes. The oxide, MnO prepared by getting MnO2 with hydrogen or posting MnCO3 with hydrogen or nitrogen. It is readily oxidized in air at ordinary temperature to form Mn2O3 or Mn2O4. All the four halides of manganese with the chemical formula MnX2 are stable compounds and isomorphous with the magnesium halides. The halides are conveniently prepared by reacting MnCO3 with the appropriate hydracids of fluorine, chlorine, bromine, iodine.
In chemistry, manganese(II) forms an extensive range of complex compounds with different types of stereochemistry. The d5 configuration of manganese offers no crystal field stabilization energy (CFSE) in high spin octahedral and tetrahedral complexes. In +2 oxidation state, it forms stable high-spin octahedral complexes are formed by ammonia, EDTA, oxalate, ethylenediamine, SCN–. The chemical equilibrium constants for the formation of these ions from aqua-ion are very low, which may again be attributed to the absence of any gain in CFSE.
Uses of Manganese
The transition element, manganese is mainly used as an additive for making steel. Manganese combines with the sulfur present in steel which prevents the formation of brittle materials like FeS. Steel alloys with 70 to 80 percent manganese are known as ferromanganese and with 15 to 20 percent of metal are called spiegeleisen. Due to the high strength, the manganese alloys have great industrial demand and is used for making railway tracks, safes, rifle barrels, crushing machines, drilling rods, and prison bars. The other alloy that contains 17 to 19 percent chromium, 8 to 10 percent manganese, 0.75 to 1 percent copper, and small amounts of carbon and silicon has high corrosion resistance and chemical attack.
Several non-steel alloys like manganese bronze alloy used for making propeller blades and manganin in electrical instruments due to their electrical resistivity. Manganese and its compounds are widely used as a chemical catalyst, in the pharmaceutical supplement, fertilizer, glass industry as a decolorizer, and in electrical dry cells as a depolarizer.