Manganese is a transition metal in Group VIIB (Group 7) in the 4th period. Other elements in the group are technetium (Tc) and rhenium (Re).
Manganese metal can be extracted from the ore pyrolusite (manganese dioxide, MnO2), and in the eighteenth century it was referred to as "black magnesia" or "manganese". The symbol Mn is an abbreviation of the name. Šelė, Bergman, and others tried in vain to isolate a metal from "black magnesia". In 1774, Ganas succeeded in reducing pyrolusite with carbon.
The name is derived from the Italian word, manganese, a corrupt form of magnesia.
The metal itself does not occur free in nature, but its compounds are widely distributed, usually as the oxides, silicates, and carbonates, with pyrolusite and psilomelane (grape-like spheres of manganese dioxide) the common ones. Its ores are found principally in the Soviet Union, India, South Africa, Ghana, and French Morocco and to a lesser extent in the United States. The recent discovery of large quantities of manganese nodules on the floor of the oceans appears promising as a new source of manganese. These nodules contain about 24% manganese together with many other elements in lesser abundance.
The metal is obtained by reduction of the oxide with sodium, magnesium, or aluminum, or by electrolysis.
3 MnO2(k) + 4 Al(k) 3 Mn(k) + 2 Al2O3(k)
It is gray-white, resembling iron, but is harder and very brittle. The pure metal exists in four allotropic forms. Manganese is reactive chemically and decomposes cold water slowly. It does, however, react rapidly with acids.
Manganese is used in large amounts to toughen and harden the steel without making it brittle. This improves the rolling and forging qualities, strength, toughness, stiffness, wear resistance, hardness, and hardenability.
Manganese is usually added in the form of ferro-manganese; any steel having between 10 and 15% is known as manganese steel. Manganese is widely used in making alloys; those with varying proportions of copper, tin, and zinc (with small amounts of other metals) form both manganese bronze and manganese brass. With aluminum and antimony, especially with small amounts of copper, it forms highly ferromagnetic alloys.
The dioxide (pyrolusite) is used as a depolarizer in dry cells for the following reason. A common dry cell battery has a Zn metal anode, carbon cathode, and contains MnO2 and NH4Cl as well. The cathode reaction is
2 NH4+(aq) + 2 e- 2 NH3(d) + H2(d)
Both products are gases, and the battery would explode if they are not removed. The ammonia is taken up by Zn2+ ion (the product of the anode reaction), while H2 is removed by the reaction
2MnO2(s) + H2(g) Mn2O3(s) + H2O
Manganese(IV) oxide is also used to "decolorize" glass that is colored green by impurities of iron. The oxide is added to glass to produce an amethyst color, and it is responsible for the color of the mineral amethyst.
The dioxide is also used in the preparation of halogens
2 NaI + 2 H2SO4 + MnO2
Na2SO4 + MnSO4 + 2 H2O + I2
and in drying black paints.
Purple-black potassium permanganate, KMnO4, is a powerful oxidizing agent and is used in quantitative analysis and in medicine.