Alkaline Earth Metals
Magnesium (Mg) is atomic number 12, has an atomic weight of 24.31 amu, and has melting and boiling points of 1,202°F (650°C) and 1,994°F (1,090°C), respectively. It was isolated (as were many other alkali and alkaline earth metals) by British chemist Humphry Davy in 1808, although its existence had been known since 1755. Magnesium, like calcium, is one of the most common elements, existing at about 23,000 parts per million in the earth's crust. Its most common mineral forms in nature are dolomite and magnesite (both carbonates), and carnallite (a chloride); relatively large amounts (about 1,200 parts per million) are also present in seawater. Asbestos is a magnesium silicate mineral, as are soapstone (or talc) and mica.
Magnesium performs a critical role in living things because it is a component of chlorophyll, the green pigment that captures sunlight energy for storage in plant sugars during photosynthesis. Chlorophyll is a large molecule called a porphyrin; the magnesium occupies the center of the porphyrin molecule. (In the animal kingdom, a similar porphyrin called heme allows hemoglobin to transport oxygen around in the bloodstream; in heme's case, however, iron rather than magnesium occupies the central place in the porphyrin.) Elemental magnesium is a strong, light metal, particularly when alloyed with other metals like aluminum or zinc, and has many uses in construction; airplane parts are often made of magnesium alloys. Some of magnesium's rare earth alloys are so temperature resistant that they are used to make car engine parts. In organic chemistry, magnesium combines with alkyl halides to give Grignard reagents, vitally important in organic chemical synthesis because of their versatility. Almost any type of organic compound can be prepared by the proper selection, preparation, and reaction of a Grignard reagent.