HAFNIUM

(Hf), chemical element (atomic number 72), metal of Group IVb of the periodic table. It is a ductile metal with a brilliant silvery lustre. The Dutch physicist Dirk Coster and the Hungarian-Swedish chemist George Charles de Hevesy discovered (1923) hafnium in Norwegian and Greenland zircons by analyzing their X-ray spectra. They named the new element for Copenhagen (in New Latin, Hafnia), the city in which it was discovered. Hafnium is dispersed in the Earth's crust to the extent of three parts per million and is invariably found in zirconium minerals up to a few percent compared with zirconium. Altered zircons, like some alvites and cyrtolites, products of residual crystallization, show greater percentages of hafnium (up to 17 percent hafnium oxide in cyrtolite from Rockport, Mass., U.S.). Commercial sources of hafnium-bearing zirconium minerals are found in beach sands and river gravel in the United States (principally Florida), Australia, Brazil, western Africa, and India. Hafnium vapour has been identified in the Sun's atmosphere. Ion-exchange and solvent-extraction techniques have supplanted fractional crystallization and distillation as the preferred methods of separating hafnium from zirconium. The metal itself is prepared by magnesium reduction of hafnium tetrachloride ( Kroll process) and by the thermal decomposition of tetraiodide (de Boervan Arkel process). Hafnium is used for fabricating nuclear-control rods because it easily absorbs thermal neutrons and has excellent mechanical properties. Hafnium produces a protective film of oxide or nitride upon contact with air and thus has high corrosion resistance. It forms alloys with iron, niobium, tantalum, titanium, and other transition metals. The alloy tantalum hafnium carbide (Ta4HfC5), with a melting point of 4,215 C (7,619 F), is one of the most refractory substances known. Hafnium is chemically similar to zirconium. Both transition metals have similar electronic configurations, and their ionic radii and atomic radii are nearly identical because of the influence of the lanthanide contraction (q.v.). The most common oxidation state in hafnium is +4, although a few trivalent compounds are known. Natural hafnium is a mixture of six stable isotopes: hafnium-174 (0.2 percent), hafnium-176 (5.2 percent), hafnium-177 (18.6 percent), hafnium-178 (27.1 percent), hafnium-179 (13.7 percent), and hafnium-180 (35.2 percent). atomic number 72 atomic weight 178.49 melting point 2,227 C (4,041 F) boiling point 4,603 C (8,317 F) specific gravity 13.31 (20 C) valence 4 electronic config. 2-8-18-32-10-2 or (Xe)4f 145d26s2