Hafnium
Hafnium is a metal element which has a bright silver luster. Its atomic symbol is Hf and its atomic number is 72. Its melting point is 4,051.4 ± 68°F (2,233 ± 20°C), its boiling point, 8,317.4°F (4,603°C), and it has an atomic weight of 178.49. Reports of its physical properties sometimes differ, probably because of the presence of zirconium as an impurity in the sample.
The discovery of hafnium was made during the early part of the twentieth century. By 1920, all but a handful of the elements had been discovered. Based on three empty spaces in Henry Moseley's version of the periodic table, chemists knew that elements 43, 61, and 72 had yet to be discovered. They also knew a great deal about the chemical and physical properties of the missing elements. Chemical theory had advanced to a point where researchers had a reasonably good idea as to where to look and what to look for in their searches for the missing elements.
In particular, they understood that element 72 would be quite different from the rare earth elements that precede it in the periodic table, and quite similar to zirconium, the element above it in the table. In fact, Niels Bohr, who discovered the relationship between electron configuration and chemical properties, at one point suggested to one of his students, György Hevesy, that he look for element 72 in the ores of zirconium.
In June 1923, working with Dirk Coster, Hevesy found element 72. The two scientists used the methods of X-ray analysis developed by Moseley to examine a zirconium ore from Norway. They observed a set of spectral lines that they had never seen before in the mineral and attributed the lines to the missing element. They named the element hafnium after the ancient name of Copenhagen, Denmark.
Hevesy later analyzed many other zirconium minerals and found hafnium in all of them. The element is actually more abundant than gold, silver, and some other better known elements. However, it is so similar to zirconium that the two metals can be separated only with the greatest difficulty.
Hafnium has three properties that make it useful in control rods used in nuclear reactors: not only does it absorb neutrons efficiently, but it is very strong and it is resistant to corrosion. Hafnium nuclear rods are used in nuclear submarines. Compounds of hafnium have also been produced and used. Hafnium boride, carbide, oxide, and nitride are all excellent refractory materials. The metal is also used in light bulb filaments and electrodes. In the United States just over 100,000 pounds of hafnium are used each year.
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