hafnium (hăfˈnēəm) [key], metallic chemical element; symbol Hf; at. no. 72; at. wt. 178.49; m.p. about 2,227°C; b.p. 4,602°C; sp. gr. 13.31 at 20°C; valence +4. Hafnium is a lustrous, ductile, silvery metal with a hexagonal, close-packed crystalline structure. Its chemical properties are almost identical to those of zirconium, the element directly above it in Group 4 of the periodic table. The two elements are among the most difficult to separate—zirconium is almost always an impurity in hafnium and affects its physical properties. Finely powdered hafnium can spontaneously ignite in air; because of this reactivity the metal has found use in the manufacture of light bulbs and vacuum tubes as a scavenger for small amounts of oxygen and nitrogen. Hafnium reacts directly with the halogens to form tetrahalides, and when heated it reacts with carbon, boron, sulfur, and silicon. Hafnium carbide is a refractory material with an extremely high melting point. Hafnium metal is produced by the Kroll process, in which a hafnium tetrahalide is reacted with magnesium or sodium metal. Because it is a good neutron absorber, hafnium metal is often used for nuclear reactor control rods. It has been alloyed with several other metals, among them iron and titanium. Hafnium is found widely distributed in nature, usually in association with zirconium minerals such as zircon. The existence of hafnium was suspected for many years before it was demonstrated (1923) through X-ray spectroscopic analysis by Dirk Coster and Georg von Hevesy. They named the element for Hafn, Latin for Copenhagen, the city where they had made the discovery.