DENSITY: 13.31g/cm 3

Hafnium was discovered in 1923 by Danish chemist Dirk Coster working together with Hungarian physicist György K. Hevesy. The electronic structure of hafnium had been predicted by Niels Bohr, and Coster and Hevesy found evidence of a substance whose pattern matched what had been predicted. The element predicted by Bohr was finally identified as being part of the mineral zircon by means of x-ray spectroscopy analysis. Due to its discovery in Copenhagen (whose ancient Latin name was Hafnia), the element was named hafnium.

Most zirconium-containing minerals are 1 to 3 percent hafnium. Hafnium is a ductile metal with a brilliant silver luster. The properties of hafnium are often difficult to ascertain, as measurements of these properties are sometimes distorted by the presence of zirconium impurities. Of all the elements, zirconium and hafnium are two of the most difficult to separate from one another. Hafnium is a group IV transition element.

Because hafnium has a high absorption cross-section for thermal neutrons (almost 600 times that of zirconium), has excellent mechanical properties, and is extremely corrosion resistant, it is used to make the control rods of nuclear reactors. It is also applied in vacuum lines as a "getter"—a material that combines with and removes trace gases from vacuum tubes. Hafnium has been used as an alloying agent for iron, titanium, niobium, and other metals. Finely divided hafnium is pyrophoric and can ignite spontaneously in air.

Melting near 3,890°C, (7,034°F), hafnium carbide (HfC) has the highest melting point of any known binary compound. Hafnium nitride (HfN) also has a very high melting point (around 3,305°C, or 5,981°F). Other hafnium compounds include hafnium tetrachloride (HfCl 4 ), hafnium tetrafluoride (HfF 4 ), and hafnium dioxide (HfO 2 ).

SEE ALSO Bohr, Niels ; Inorganic Chemistry ; Zirconium .

Herbert W. Roesky

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