Magnesium
Magnesium
Magnesium (Mg) is one of the most abundant structural metals (those metals used to build things), ranking third behind iron and aluminum. Magnesium compounds are found in mineral rocks such as dolomite and magnesite, while the Earth’s oceans contain a practically unlimited supply of the metal in the form of dissolved magnesium chloride. One cubic mile of sea-water holds some six million tons of the element. The lightest of all the structural metals, magnesium (atomic number 12) is about one-third lighter than aluminum and gives more strength and stiffness per pound than any other common metal.
Properties
Magnesium is a moderately hard, silvery white metal, the lightest of all structural metals. The element is easily molded, bent, cut, and otherwise fabricated. Magnesium’s melting point is about 1,204°F (651°C), its boiling point is around 1,994°F (1,090°C), and its density is 1.005 ounces per cubic inch (1.738 grams per cubic centimeter). It is a solid at room temperature.
Magnesium is an active metal that reacts slowly with cold water and more rapidly with hot water. It reacts with oxygen at room temperature to form a thin skin of magnesium oxide (MgO) that protects the underlying metal from further oxidation. At higher temperatures, magnesium reacts vigorously with oxygen to produce a blinding white light. The metal also reacts with most acids and with some strong alkalis. It combines easily with many non-metals, including nitrogen, sulfur, phosphorus, and the halogens. It also reacts with a number of compounds, such as carbon monoxide (CO), carbon dioxide (CO2), sulfur dioxide (SO2), and nitric oxide (NO).
Occurrence and Extraction
The abundance of magnesium in the Earth’s crust is estimated to be about 2.1%, making it the sixth most common element in Earth. Its estimated crustal abundance is 0.37 ounces per pound (2.33 x 104 milligrams per kilogram). It also occurs in seawater, usually as magnesium chloride (MgCl2). Its estimated oceanic abundance is 0.17 ounces per gallon (1.29 x 103 milligrams per liter). According to some estimates a cubic mile of seawater may contain up to six million tons of magnesium metal.
Magnesium compounds such as Epsom salts were first prepared in the late 1600s and early 1700s. For a while, the oxide compound (magnesia) was confused with lime (calcium carbonate), until Scottish physicist and chemist Joseph Black (1728–1799) discovered the difference between the two substances in 1754. English chemist Sir Humphry Davy (1778–1829) demonstrated in 1808 that magnesia was the oxide of a new metal, which he named magnesium, but the element was not isolated until twenty years later, when French chemist Antoine Alexandre Brutus Bussy (1794–1882) combined dry magnesium chloride with potassium to produce the metal in its pure form. Using electrolysis, English physicist and chemist Michael Faraday (1791–1867) obtained magnesium from magnesium chloride in the 1830s. Then, in 1852, German chemist Robert Wilhelm Bunsen (1811–1899) developed an electrolytic cell for producing large quantities of magnesium. For the next sixty years, German chemists further developed Bunsen’s cell and pioneered in the production and use of magnesium. The element is relatively easy to extract from seawater by electrolysis, which splits the chloride compound and produces magnesium metal and chlorine gas. By 1909, Germany was manufacturing magnesium commercially, and the United States began producing magnesium during World War I, eschewing German imports.
During the war, magnesium was used in incendiary bombs, which ignite and burn upon impact, as well as flares and tracer bullets. Powdered magnesium burns with a dazzling white flame, a property of the metal Bunsen had demonstrated in the 1850s. This quality has been exploited in many other products, such as fireworks, and the metal is used in photography to provide a brilliant flash for lighting purposes.
Magnesium’s light weight is its most valuable attribute, however. When World War II began in 1939, Germany was well-positioned to use magnesium for aircraft construction and other military applications. In response, U.S. companies stepped up their magnesium manufacturing efforts, resulting in a dramatic increase in magnesium production in the United States; it peaked in 1943 at nearly 184,000 tons. In addition to electrolytic extraction of magnesium from seawater, the element can be produced from mineral rocks by various thermal processes. These methods use a reducing agent such as ferrosilicon to break down magnesium compounds and form vapors of magnesium, which are then distilled into crystals and melted.
Today, many automotive manufacturers in Europe and the United States are using or testing vehicle parts made of magnesium alloys—mixtures of magnesium and other metals, most often aluminum. Magnesium’s light weight will become more critical as automakers try to meet new vehicle mileage standards. Because magnesium is easily cast into complex structures, automakers could also reduce the number of parts needed and streamline the assembly process. Some manufacturers already use magnesium alloys in vehicle bodies and frames.
Magnesium alloys are also used in a host of other products, such as sports equipment, for which light weight is an advantage. Baseball catchers’ masks, skis, racecars, and horseshoes are made with magnesium alloys. Consumer goods such as ladders, portable tools, electronic equipment, binoculars, cameras, furniture, and luggage also benefit from magnesium’s light weight. Other applications make use of its ability to absorb vibration. Magnesium is an important element from a biological standpoint as well. Scientists have learned that magnesium ions aid in the digestive process, and several magnesium compounds are active ingredients in medicines such as milk of magnesia (magnesium hydroxide) and Epsom salts (magnesium sulfate). In plants, magnesium is even more critical—it is a component of chlorophyll, the green pigment that converts the sun’s energy, in the form of light, into food. Magnesium’s position in the chlorophyll molecule, which is similar to that of iron in hemoglobin, was discovered by Richard Willstätter in the early 1900s. This was the first clue to magnesium’s importance as a plant nutrient; before, scientists thought that it was an impurity. Since then, agricultural fertilizers for magnesium-deficient soils have greatly increased crop yields.
Health Issues
Magnesium is essential for good health in both plants and animals. It forms part of the chlorophyll molecule that catalyzes the conversion of carbon dioxide and water to carbohydrates in green plants. Plants that do not get enough magnesium do not manufacture an adequate amount of chlorophyll, and their leaves tend to become yellow.
Magnesium also occurs in enzymes needed by animals. The amounts required are so small that magnesium deficiency diseases are rare. In some areas, however, people with poor diets may not get the magnesium they need, producing symptoms such as extreme agitation or aggressiveness.
Overexposure to magnesium is also rare, but may occur and, if it does, may produce health problems. For example, people who work around magnesium metal may inhale the fumes, experiencing symptoms such as irritation of the throat and eyes, damage to muscles and nerves, and loss of feeling and paralysis.