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Metallic chemical element, one of the transition elements, chemical symbol Au, atomic number 79. It is a dense, lustrous, yellow, malleable precious metal, so durable that it is virtually indestructible, often found uncombined in nature. Jewelry and other decorative objects have been crafted from gold for thousands of years. It has been used for coins, to back paper currencies, and as a reserve asset. Gold is widely distributed in all igneous rocks, usually pure but in low concentrations; its recovery from ores and deposits has been a major preoccupation since ancient times (see cyanide process). The world's gold supply has seen three great leaps, with C. Columbus's arrival in the Americas in 1492, with discoveries in California (see gold rush) and Australia (1850-75), and discoveries in Alaska, Yukon (see Klondike), and S. Africa (1890-1915). Pure gold is too soft for prolonged handling; it is usually used in alloys with silver, copper, and other metals. In addition to being used in jewelry and as currency, gold is used in electrical contacts and circuits, as a reflective layer in space applications and on building windows, and in filling and replacing teeth. Dental alloys are about 75% gold, 10% silver. In jewelry, its purity is expressed in 24ths, or karats: 24-karat is pure, 14-karat is 58.3% gold, etc.


Metallic chemical element, one of the transition elements, chemical symbol Pt, atomic number 78. A very heavy, silvery-white precious metal, it is soft and ductile, with a high melting point (3,216°F, or 1,769°C) and good resistance to corrosion and chemical attack. Small amounts of iridium are commonly added for a harder, stronger alloy that retains platinum's advantages. Platinum is usually found as alloys of 80-90% purity in placer deposits, or more rarely combined with arsenic or sulfur. It is indispensable in high-temperature laboratory work, for electrodes, dishes, and electrical contacts that resist chemical attack even when very hot. Platinum is used in dental alloys, and surgical pins. Known as "white gold," it is used in expensive jewelry and is far more costly than gold. The international primary standards for weight and length are 95% platinum, 5% iridium. An alloy of 76.7% platinum and 23.3% cobalt forms the most powerful permanent magnets known.


Titanium is a lustrous silver-white metal that exhibits allotropy ; below about 880 it has a hexagonal crystalline structure, but above that temperature it changes to a cubic crystalline structure. The metal is strong and has low density; it is ductile when pure and malleable when heated. Its chemical properties resemble those of zirconium, the element below it in group IVb of the periodic table . When heated, it ignites and burns in air. It is the only element that burns in nitrogen. It is very corrosion resistant and is unattacked by most acids, by moist chlorine gas, or by common salt solutions. Several of its compounds are commercially important. Pure crystalline titanium dioxide (titania) is used as a gemstone. The dioxide is also widely used as a paint pigment, especially for exterior paints. Titanates are formed from the dioxide, which is weakly acidic. An interesting example is barium titanate, which is piezoelectric and can be used as a transducer for the interconversion of sound and electricity. Titanium tetrachloride, a liquid, fumes in moist air; it is used for smoke screens and in skywriting. It is also an important catalyst in the polymerization of olefins. Titanium esters, formed by the reaction of the tetrachloride with alcohols, are used as waterproofing agents on fabrics. Titanic sulfate is used as a textile mordant. Titanium metal and its alloys are light in weight and have very high tensile strength, even at high temperatures. These metals are utilized in aircraft and spacecraft construction and in naval ships, guided missiles, and lightweight armor plate for tanks. Titanium compounds are widely distributed in nature. Rutile, the native dioxide, and ilmenite, which contains, besides titanium, iron and oxygen, are its chief sources. The metal cannot be produced by reduction of the dioxide, because titanium reacts with both oxygen and nitrogen at high temperatures. One method used consists in passing chlorine over ilmenite or rutile, heated to redness with carbon. Titanium tetrachloride, which is formed, is condensed, purified by fractional distillation, and then reduced with molten magnesium at 800 in an atmosphere of argon. Titanium is present in the sun and certain other stars, in meteorites, and on the moon. Titanium dioxide causes the star effect in certain sapphires and rubies. The element was discovered (1791) by William Gregor and rediscovered (1795) by M. H. Klaproth, who gave it its present name.