Manganese

manganeso

Manganese, like magnesium, is named after the Greek region of Magnesia, where there are many manganese ores, including the colorful rodochrosite mineral. The pure form of the metal is obtained mainly from the pyrolusite mineral. Pure manganese is dense, hard and brittle. This element is present in seawater in the form of manganese hydroxide and manganese oxide, which have accumulated in layers over millions of years to form masses on the seabed. The human body needs a small amount of manganese, which we can obtain from mussels, nuts, oats and pineapples. Applications of manganese include its use in strengthening steel, which is used in the manufacture of railroad tracks and tank armor. Certain dry cell batteries carry a mixture containing manganese oxide. Manganese compounds are also added to gasoline and are used to clean impurities from glass to lighten it or give it a purple color. In prehistoric times, manganese dioxide, MnO2, was crushed to make the dark colors used in cave paintings.

Chemical properties

Manganese is an essential element in animals: it increases bone strength, helps absorb vitamin B1 and is an important cofactor for enzymes.

Manganese metal is readily oxidized in the presence of oxygen gas, water, and hydrochloric acid. For example, in hydrochloric acid, the following reaction takes place:

Mn(s) + 2HCl(aq) → MnCl2(aq) + H2(g)

In compounds, manganese can be in any of the oxidation states from “+2” to “+7,” inclusive. Examples of manganese in these states are illus- trated in the following series: Mn «0»; Mn2+ «+2»; Mn3+ «+3»; MnO2 «+4»; MnO3 «+5»; MnO42- «+6»; MnO4 «+7».

As it is in group VIIA, its "+7" oxidation state is the preferred one.

The manganic ion (Mn3+) tends to be unstable in aqueous solution and disproportionates into Mn(II) and Mn(IV), as shown in the following reaction:

2Mn3+(aq) + 2H2O(l) → Mn2+(aq) + MnO2(s) + 4H+(aq)

Disproportionation means that an element initially in only one oxidation state changes to two products: one product exhibiting a lower oxidation state and the other exhibiting a higher state.

The oxides formed in the “+2” and “+3” states (manganous oxide (MnO) and manganic oxide (Mn2O3), respectively) are bases due to their solubility in acid solutions, as shown in the following example:

MnO(s) + 2HCl(aq) → MnCl2(aq) + H2O(l)

In the “+4” state, manganese forms black manganese dioxide (MnO2), which is the only compound of manganese (IV). Manganese dioxide is amphoteric because it is soluble in both acid and basic solutions. In higher oxidation states, oxides of manganese are acidic because they dissolve in basic solutions.

The manganate ion (MnO42-) disproportionates in acidic solution to the “+4” and “+7” states, as shown in the following reaction:

3MnO42-(aq) + 4H+(aq) → MnO2(s) + 2MnO4 (aq) + 2H2O(l)

where MnO4 is the purple-colored permanganate ion in which manga- nese is in the “+7” oxidation state.

In acidic solution, permanganate also oxidizes the manganous ion (the “+2” state) to form manganese dioxide (the “+4” state), as shown in the following equation:

2MnO4(aq) + 3Mn2+(aq) + 2H2O(l) → 5MnO2(s) + 4H+(aq)

In this reaction, manganese begins in two different oxidation states and forms a product that exhibits an oxidation state intermediate between the two initial states.

Summary of properties (Mn)

Atomic weight54.938043(2)
Discoverer (year)Gahn, Johan Gottlieb (1774)
Natural formmetallic solid (body centered cubic)
Electron configuration[Ar] 3d5 4s2
Melting point (ºC)1245
Boiling point (ºC)1962
Abundance in earth's crust (ppm)950
Isotope (abundance)55Mn (100)
Density g/cm37.43
Van der Waals radius (pm)205
Covalent radius (pm)129
Electronegativity (Pauling)1.83
Vaporization enthalpy (kJ/mol)219.70
Enthalpy of fusion (kJ/mol)12.91
Specific heat capacity (J/g·K) at 25ºC and 1 atm0.48
Thermal conductivity (W/cm·K) at 25ºC and 1 atm0.080
Oxidation state+7, +4, +3, +2
Electron affinity (eV)unstable ion
1st Ionization potential (eV)7.4340

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