chemical of the month Hafnium W. Conard Fernelius Kent State University Kent, OH 44242
Hafnium is unique among the elements, not only for its set of physical properties and chemical behavior but also because of the role its discovery played in establishing some important generalities of inorganic chemistry. The periodic table of the second decade of this century differed significantly from that we know today. Chemists then stated the periodic law in terms of atomic weights because atomic numbers had not yet been estahlished. While chemists had learned to live with the inversion of increasing atomic weights in the cases of argon and potassium, cobalt and nickel, and tellurium and iodine, there were many blank spaces which most expected would be occupied hy elements yet to he discovered. The situation was particularly distressing because there was no way a t that time to estimate how manv rare earths there were nor whether all of them should he crowded into one space (directly helowyttrium in the periodic table) or whether the group of unusually closely related elements extended over not one, but two spaces in the periodic table. Until the number of possible rare earths could be estahlished, one really could not say how many elements there should be between hvdroren, element. . - the lirhtest and uranium, the heaviest. The work of Moselev on the characteristic X-rav swectra of the elements alteredtbis picture dramatically. 1fthe atom of hydrogen had an atomic number of 1and that of uranium 92, then.there should be 90 elements between them. Further, between the elements barium, atomic numher 56, and tantalum, atomic number 73, there should be 16 elements. Were all these rare earths? Urbain had announced the isolation of a new element, celtium, and claimed on the basis of chemical behavior that it was the heaviest rare earth. On the other hand, Bohr's theory of the distribution of electrons around nuclei, which was gaining wide acceptance, predicted that element 72 should be a congener of zirconium rather than a rare earth. Both Bohr and Urhain could not be right. On the basis of Moseley's work, one could predict the X-ray soectrum of an element. Therefore. Urhain called uoon Moelement 52in the X-ray spectrum of the sample sent to him. There was good reason to he a hit suspicious about zirconium. Determinations of its atomic weight did not agree despite extreme care in purifying the compo'mds used in the determination. Since it was important to Bohr to establish that element 72 was ekazirconium, he encouraged two workers in his laboratory, D. Coster and G. von Hevesey, to search for element 72. They carefully fractionally crystallized (NH&ZrFn . - - and found that a small amount of anew element actually accompanied zirconium. They were able to concentrate this element and eventuallvisolate it in wure form. Thev called it hafnium from Hafnia, an old name for Copenhagen.
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Journal of Chemical Education
edited by DARRELL ti BEACH The Culver Academies Cuiver, Indiana 4651 1
In the course of this work, they developed a new method of analysis-X-ray spectrographic analysis. To determine the amount of element 72 present in a sample, they added a known amount of tantalum. element 73. and comnared the intensities of the corresponding lines for the two elements. Hafnium is a constant comoanion of zirconium in minerals (about 2%) except in two rare minerals where the hafnium constant may reach 3 5-5.090. Why had element 72 gone unrecognized so long? The reason became evident as the chemical hehavior of hafnium was es tahlished. Workers in the field of the rare earths had become accustomed to small differences between neighboring elements, but nowhere had anyone encountered such small dlfferences between two elements as those between zirconium and hafnium. If the similarity of properties between two adjacent rare earths was such as to iustifv calling them twins. then that between zirconium and hafnium w& sufficiently close to justify calling them identical twins. Why? If one seeks a single property of related atoms or ions by which differences in hehavior can he readily understood, that property is atomic or ionic size. The size of the ions of adjacent rare earths differ on the average by 7.6%while that between zirconium and hafnium is only 1.5%. As one progresses through the series of rare earths, each additional electron adds not to the outer or next inner shell of the atoms but to the antipenultimate ( f ) shell, deep within the atoms. The increasine charge on the nucleus and the oroximitv of the electrons in this shell to the nuclei result; in strong attraction which decreases the size of the atoms progressively through the rare earths. This so-called lanthanide contraction is responsible for contracting the size of hafnium to slightly less than that of zirconium. (This contraction is evident in the wairs Ta and Nb and in W and Mo althourh . the effect is not so dramatic as between Hf and Zr.) As long as one considers only chemical properties, the slight difference between hafnium and zirconium provides little stimulus to develop the chemistry of hafnium as distinct from that of zirconium. However, nuclei of atoms of related elements differ in respects other than size and weight. Zirconium is almost transparent to neutrons; whereas, hafnium strongly absorbs them. Hence when zirconium was deemed to he an ideal material for the cladding of fuel rods in atomic reactors, it became essential to remove the hafnium from zirconium or, rather. to seuarate as much oure zirconium from a mixture as could be done economically.'~eparation techniques have been developed so that today one can have the natural mixture of the two elements, pure zirconium, or a zirconium enriched in hafnium, and all of these at compartively low cost. Hafnium is an element with a distinguished history!
The structure, properiies, and uses of a variety of chemicals are highlighted in this feature. which is aimed at increasing the use of descriptive chemistry.
