lanthanum (lănˈthənəm) [key] [Gr., = to lie hidden], metallic chemical element; symbol La; at. no. 57; at. wt. 138.90547; m.p. about 920°C; b.p. about 3,460°C; sp. gr. 6.19 at 25°C; valence +3. Lanthanum is a soft, malleable, ductile, silver-white metal; at room temperature it has a hexagonal close-packed crystalline structure that is unstable at higher temperatures (see allotropy). Lanthanum is usually considered the first member of the lanthanide series, a group of elements with similar physical and chemical properties. It is one of the rare-earth metals of Group 3 of the periodic table. Lanthanum is a chemically active element. It oxidizes rapidly in air and reacts with water to form the hydroxide. It reacts readily with acids, with elemental boron, carbon, nitrogen, phosphorus, selenium, silicon, or sulfur, and with the halogens. The oxide and the boride are used in electronic vacuum tubes; the oxide is added to optical glass to increase its alkali resistance and refractive index. Its aluminate is a perovskite ceramic used as a superconductor substrate. Although lanthanum is not found uncombined in nature, it occurs in the rare-earth minerals monazite and bastnasite. Lanthanum may be prepared by reduction of lanthanum fluoride with calcium metal. Lanthanum is used in rechargeable nickel-metal hydride batteries. It may be used in making ductile cast iron; alloyed with other metals, it is used in cigarette lighter flints. Natural lanthanum is a mixture of two stable isotopes. One radioactive byproduct of the fission of plutonium, thorium, or uranium is a mixture of radioactive isotopes of lanthanum; 17 radioactive isotopes are known. Lanthanum was discovered in the form lanthanium oxide, called lanthana, in 1839 by C. G. Mosander.