titanium (tĪtāˈnēəm, tĭ–) [key] [from Titan ], metallic chemical element; symbol Ti; at. no. 22; at. wt. 47.867; m.p. 1,675°C; b.p. 3,260°C; sp. gr. 4.54 at 20°C; valence +2, +3, or +4. Titanium is a lustrous silver-white metal that exhibits allotropy; below about 880°C 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 4 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°C 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.