Name: From the Latin Hafnia, meaning Copenhagen.
Atomic mass: 178.49.
History: Discovered in 1923 by Danish chemist Dirk Coster and Hungarian chemist Georg Karl von Hevesey in Copenhagen.
Occurrence: Rare. It occurs as a 1­5% impurity in all zirconium ores and is generally obtained as a by-product of zirconium refining. Found in Australia, Brazil, Sri Lanka, and the U.S.
Appearance: Lustrous, silvery, solid metal.
Behavior: Resists corrosion as a solid due to an oxide film on its surface. Burns in air as a powder. Poorly absorbed by the body, so is of low toxicity.
Uses: Hafnium is used in nuclear control rods, high-temperature alloys, and ceramics.
Until recently, philosophers of science have usually concentrated their attention on physics and biology while carefully avoiding chemistry. One major exception seems to have been a philosopher who is arguably the most outstanding among them, namely Sir Karl Popper. Popper often mentioned chemical issues, including an article in which he referred to the discovery of element 72, or hafnium as it subsequently became known.

This element later took on a special significance for me as a Ph.D. student working on a thesis in the philosophy of chemistry. More important, it led to my meeting Popper at his home in London and my having a three-hour audience with this great man at a stage in his life when he had gone into seclusion. But before returning to my meeting with him, let me say something about hafnium and the scientific issue at stake.

Following Henry Moseley's establishment of an experimental technique for placing the elements in a definite sequence in 1914, scientists realized that precisely seven elements remained to be discovered. One of them was element 72. Gradually, some of these missing elements were isolated, but element 72 remained elusive. The story of the eventual discovery of element 72 has been told many times, but it is almost invariably incorrect.

According to the popular story, the chemists of the day believed that hafnium would be a rare-earth element. Meanwhile, the physicist Niels Bohr, who first applied the quantum theory to study atoms and the periodic system, is usually credited with having correctly predicted that hafnium would in fact be a transition element. Moreover, he is supposed to have instructed his assistants Dirk Coster and Georg Karl von Hevesy to search for the element among the ores of zirconium, where they indeed discovered it.

The episode is taken as an early vindication of the quantum theoretical approach to the explanation of the periodic system. Unfortunately, this popular account is incorrect. And yet it was this version that Popper used many years later to make an even grander claim that this represented the best illustration of the reduction of chemistry to quantum theory.

Meanwhile, I was researching the question of the reduction of chemistry to quantum theory and was being advised by the son of the radiochemist Fritz Paneth, who provided me with some of his father's scientific correspondence. These documents showed that it was not Bohr who had suggested to his assistants that they might look for hafnium in the ores of zirconium. The suggestion had actually been made by the elder Paneth, a chemist with little interest in quantum theory, through purely chemical arguments.

Upon further investigation, I found out that not all chemists had in fact expected that hafnium would be a rare-earth element [Ann. Sci., 51, 137 (1994)]. For example, Julius Thomsen, a Danish chemist who is known to have influenced Bohr, was one of the first to correctly predict, as early as 1895, that element 72 would be a transition metal. Similarly, the English chemist C. R. Bury in 1921 not only predicted the chemical nature of this element but even published its correct electronic configuration before Bohr ventured to do so in 1923.

CROSS-HATCHED A heat-tinted hafnium-crystal bar.
What Bohr had done was to rationalize the electronic configuration of hafnium, while implying that he was calculating it deductively from the theory. I'm not implying any scientific misconduct here, but perhaps an understandably exaggerated account of the power of the newly developed theory by its principal architect.

The revised version of the story is of interest to philosophers of science, who are frequently concerned with the extent to which any given theory rigorously predicts phenomena or merely accounts for the facts that are already known. It appears that in many respects Bohr's theory of the periodic system was of the latter kind. When I pointed this out to Popper, he was quick to accept my arguments before going on to share his recollections of people like Erwin Schrödinger and Ludwig Wittgenstein, with whom he had personally interacted. Throughout our meeting, Popper would frequently leap to his feet to go in search of some articles on the origin of life, a question that very much inspired him in the final years of his long and productive career.

But I will always be grateful to element 72, since it was due to the story of its discovery that I had the opportunity of meeting perhaps the greatest philosopher of science of the modern era.

Eric R. Scerri is a lecturer in the chemistry and biochemistry department at the University of California, Los Angeles. He is also a leading researcher in the history and philosophy of chemistry and the editor of the journal Foundations of Chemistry, http://www.kluweronline.com/issn/1386-4238.

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