praseodymium (prāˌzēōdĭmˈēəm, –sēō–) [key] [Gr., = green twin], metallic chemical element; symbol Pr; at. no. 59; at. wt. 140.90765; m.p. 931°C; b.p. 3,512°C; sp. gr. about 6.8; valence +3 or +4. Praseodymium is a soft, malleable, ductile, silver-yellow metal. It exhibits allotropy; the α-form (hexagonal crystalline structure) has the density given above, but the β-form (above 800°C, body-centered cubic crystalline structure) is less dense. Praseodymium is a rare-earth metal of the lanthanide series in Group 3 of the periodic table. When exposed to air it forms a green oxide that does not protect it from further oxidation. Although the pure metal may be prepared by reduction of the chloride, it has few commercial uses. A major use of the metal is in a pyrophoric alloy used in cigarette lighter flints, but it need not be purified for this application. Praseodymium compounds have many uses. The oxide is used in carbon electrodes for arc lighting. The salts are used to color enamels and glass. Didymium glass used in glassblower's goggles contains praseodymium; this glass absorbs the yellow sodium glare of light from the torch flame. The major commercial source of praseodymium is the rare-earth minerals monazite and bastnasite. Praseodymium was discovered in 1885 by C. A. Von Welsbach, who separated Mosander's "didymium" into two components, the earths neodymia and praseodymia.