Calcium is a soft, silvery-white metal. It is the fifth most abundant element, making up about 3.5 per cent of the earths crust. It is also the most common (and cheapest) alkaline earth element in both occurrence and use. It occurs naturally in a wide range of forms, being found in chalk, marble, limestone, and the shells of mollusks and crustaceans.
Immense quantities of calcium compounds are used in building and construction. For many centuries, limestone has been heated to give carbon dioxide and quicklime, which is converted with water to slaked lime and used in mortar and brickwork. Calcium compounds are also the basis of cement and plaster and are used in leather tanning and petroleum refining and in making fertilizer, paint, and a wide variety of other products.
Calcium is essential to living organisms. It is necessary for the growth and maintenance of teeth and bones. It also helps control muscle action and blood clotting and aids in transmit-
Several alkaline earth elementsare used in fireworks(left), flares, and other pyrotechnic devices. Alkalines burn easily and produce an intense light. Moreover, each element (and its compounds) emits light of a characteristic color. For example, the white colors in the firework display were produced by magnesium, the red colors by strontium, and the green by barium.
The shellsof many mollusks—such as the helmet cowrie (Cypraecassis rufal fbe/ow/~d.n<j crustaceans consist largely of calcium carbonate. This compound is secreted by the underlying tissues to form the shell. Calcium salts also play an important role in other animals. For example, they help to form bones and teeth in humans and other vertebrates.
Major groups of elements: Alkaline earths 31
ting "messages" along the body's nervous system. Too little calcium causes rickets (a softening of the bones) and failure of the blood to clot. Too much calcium causes a hardening of joints and the formation of kidney stones.
Strontium is a soft, silvery metal that is found in the minerals celestite and strontian-ite. Outside of one compound that is used in fireworks and flares, it has few industrial applications, calcium or barium being cheaper. Strontium 90, found in the debris of certain atomic explosions, is a hazardous, radioactive isotope. This means that the isotope, which is a variation of an element in which the number of neutrons in the atom varies, gives off large amounts of energy in the form of heat, light, or electricity. The radioactivity of strontium 90 destroys the tissues that produce blood in people and animals.
Barium is a soft, heavy, silver-colored metal that is found in several minerals. It has various, though limited, uses. Different compounds are used in manufacturing magnets, flares, electrical equipment, paints, and paper. One compound, barium sulfate, is used in making internal X-ray examinations.
Radium, a silver-white metal, and its compounds are highly radioactive. They are found mainly in uranium and thorium ores. The radioactive decay of uranium is constantly producing isotopes of radium. These isotopes are also unstable and keep breaking down, eventually forming a stable isotope of lead. Once used in treating cancer and in making fluorescent paint, radium has been replaced by cheaper and safer substitutes. Its radioactivity destroys red blood cells and can cause bone cancer.
Hang glidersoften use magnesium-aluminum alloy for their frames because of its combination of light weight and strength.