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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 seawater 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.

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 Joseph Black discovered the difference between the two substances in 1754. Humphry Davy 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 Bussy (1794-1882) combined dry magnesium chloride with potassium to produce the metal in its pure form. Using electrolysis, Michael Faraday obtained magnesium from magnesium chloride in the 1830s. Then in 1852, Robert Bunsen 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 America 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, American 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 America 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, race cars, 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, and 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.

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