Manganese (Mn)

• Element Symbol: Mn
• Atomic Number: 25
• Atomic Mass: 54.938
• Group # in Periodic Table: 7
• Group Name: Transition metals
• Period in Periodic Table: 4
• Block of Periodic Table: d-block
• Discovered by: Johan Gottlieb Gahn (1774)

Manganese is one of the earliest elements to be used by humans, who first used it as a pigment to paint pictures of animals and people on the walls of caves. Those paintings are estimated to be between 24,000 and 30,000 years old. Ancient Egyptian and Roman glassmakers used a compound they called "glassmakers’ soap" both to add color to glass and to remove the green color left by iron. Glassmakers’ soap is now known as manganese dioxide.

Manganese is named for one of the dark ores found in the region of Greece known as Magnesia. Other ores found in the region contain both the element magnesium and also the iron ore magnetite. Manganese dioxide was regularly used in alchemists’ experiments. Early chemists also used it to develop a process that produces chlorine and hypochlorite as bleaching agents. Chemists working with the compound realized that it contained an unidentified element in it, but they were not able to isolate it. In 1774 the Swedish chemist Johan Gottlieb Gahn was finally able to isolate an impure sample.

Physical Properties

Manganese is a pinkish-gray, shiny metal. At 298 kelvins (K) it is a solid, which is the element’s standard state. It is hard—its Mohs scale rating is 6—but also very brittle. It is so brittle that in its pure form it cannot be bent, cut, or shaped by machines. A transition element in the iron group of metals, which are found in the center of the periodic table, it is a good conductor, with an electrical conductivity of 2.3 × 10⁷ siemens per meter (S/m). It has a thermal conductivity of 160 watts per meter-kelvin (W/m·K). Interestingly, it was difficult for chemists to discover the difference between manganese and iron because the two elements have very similar properties and are often found together in Earth’s crust. Manganese has a high melting point of 1246 degrees Celsius (°C); its boiling point is 2061 °C.

Chemical Properties

Manganese is considered to be a moderately active metal chemically. It is active enough that it cannot be found as a free element on Earth. Powdered manganese will burn in oxygen, giving off a bright white light, and it reacts slowly with cold water but more rapidly with hot water or steam; it also dissolves in most acids, releasing hydrogen gas in the process. It also reacts with fluorine and chloride to make manganese difluoride and manganese dichloride.

Manganese rusts like iron when placed in oxygenated water. It dissolves in dilute acids. Manganese can form many different kinds of chemical bonds because it can be found in all oxidation states, from −3 to +7. The most common oxidation states of manganese are all positives: +2, +3, +4, +6, and +7. Its most stable oxidation state is +2. In this state the element has a pale pink hue.

Manganese has one naturally occurring stable isotope and eighteen radioisotopes. The most stable of the radioisotopes, manganese-53, has a half-life of 3.7 million years.

Applications

Manganese serves a vital role in the production of iron and steel. It helps make these two metals less brittle and more corrosion resistant. This is why it plays such an important role in the process of making stainless steel. It also has many other applications when it is alloyed with aluminum.

The oxides and hydroxides of manganese are commonly found in soil. They are so plentiful that manganese is the twelfth-most abundant element in Earth’s crust (0.11 percent). Manganese is mostly found near iron deposits. The most commonly found ores are pyrolusite and rhodochrosite. The majority of these ores (80 percent) are found in South Africa, but large-scale mining operations are also carried out in Australia, Georgia, Gabon, Russia, and Ukraine. It is estimated that up to 1.5 trillion metric tons of manganese nodules lie on the floor of the ocean and in deep lakes around the world. At this time no cost-efficient method has been developed to collect them.

Manganese compounds are also used as catalysts, disinfectants, fertilizers, and pigments; they also turn up as additives in gasoline to increase octane rating. Manganese dioxide is used as one pole in both zinc-carbon and alkaline batteries.

Manganese plays an important role in the metabolism of most living organisms and is present in the mitochondria of all plant and animal cells. The mitochondria are called the powerhouse of the cell. In plants they are involved in such vital processes as photosynthesis, respiration, and nitrogen assimilation. This is why fertilizers used in farming contain manganese. Animals absorb this element in the food they eat. In animals, including humans, manganese is essential to normal growth and development as well as to the maintenance of bones, skin, and the nervous system. It affects the regulation of blood sugar levels and helps prevent damage caused by the formation of molecules known as free radicals. Manganese is a cofactor in many enzymes that catalyze vital processes.

Manganese is a micronutrient. This means that a human body only needs a small amount of it to stay healthy. The recommended dose is 2.3 milligrams (mg) per day. People who do not eat enough manganese can have high blood pressure, heart ailments, malformed bones, vision and hearing problems, tremors, and severe memory loss. In most areas manganese deficiency is usually rare since the element is so available in the water supply and through a normal diet.

It is very dangerous to exceed the maximum recommended dose of any micronutrient. Too much manganese in the body can lead to severe side effects. The symptoms of an overdose include loss of appetite, headaches, leg cramps, rigid muscles, tremors, convulsions, extreme irritability, and hallucinations.

Compounds containing manganese can be toxic. Exposure to manganese dusts and fumes should not exceed 5 milligrams per cubic meter (mg/m³) even for short periods. Manganese poisoning can cause impaired motor skills and cognitive disorders. As early as 1837, British researcher James Couper found that mine workers who were regularly exposed to large amounts of manganese developed a neurological disorder called "manganism." In its early stages patients experience depression, mood swings, compulsive behaviors, and psychosis. In the later stages the disorder resembles Parkinson’s disease. Patients experience weakness, show signs of slowed speech, and talk in a monotone. An expressionless face, tremors, a forward-leaning gait, an inability to walk backward without falling, rigidity, and general problems with dexterity, gait, and balance are additional symptoms. Unfortunately, these symptoms persist even after the levels of manganese in the body drop. Studies show that long-term exposure to low doses of manganese from drinking contaminated water has been linked to childhood development disorders, such as diminished IQ and behavioral problems.

Bibliography

Chalmin, Emilie, Michel Menu, and Colette Vignaud. "Analysis of Rock Art Painting and Technology of Palaeolithic Painters." Measurement Science and Technology 14.9 (2003): 1590–97. Print.

Gray, Theodore. The Elements: A Visual Exploration of Every Known Atom in the Universe. New York: Black Dog, 2009. Print.

Parsons, Paul, and Gail Dixon. The Periodic Table: A Visual Guide to the Elements. New York: Quercus, 2014. Print.