niobium (nĪōˈbēəm) [key], metallic chemical element; symbol Nb; at. no. 41; at. wt. 92.90638; m.p. about 2,468°C; b.p. 4,742°C; sp. gr. 8.57 at 20°C; valence +2, +3, +4, or +5. Niobium is a rare, soft, malleable, ductile, gray-white metal with a body-centered cubic crystalline structure. In its physical and chemical properties it resembles tantalum, the element below it in Group 5 of the periodic table. At normal temperatures it is insoluble in solutions of most acids or alkalies. It reacts readily at high temperatures with oxygen, carbon, the halogens, nitrogen, and sulfur; it must be placed in a protective atmosphere when processed. It forms four oxides; the pentoxide, Nb2O5, is the basis of a series of salts called niobates. Niobium is important in the production of high-temperature-resistant alloys and special stainless steels; large amounts of niobium have been used in the U.S. space program. Niobium carbide is used in cutting tools. The name niobium was officially adopted by the International Union of Pure and Applied Chemistry (IUPAC) in 1949; the name columbium (symbol Cb) was used for the element in some countries prior to that and continued to be used by many metallurgists and commercial producers of the metal. Niobium is widely distributed in nature; it is about one and a half times as abundant as lead. It occurs in the minerals columbite and tantalite, together with tantalum. Separating niobium and tantalum is difficult. The element was discovered in 1801 by Charles Hatchett and first isolated in 1864 by C. W. Blomstrand.