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The classification of the chemical elements that is displayed in any of the conventional versions of the so-called Periodic Table is an expression of the Periodic Law. The latter can be rationalized easily in terms of the distribution of electrons in the energy levels of the atom. But the Periodic Law was discovered in 1869-1871 by Dmitri I. Mendeleev, before the existence of electrons had been demonstrated experimentally. How did Mendeleev manage to gain this insight, which today may seem commonplace but was extraordinary in its time?
It is clear that Mendeleev got a considerable amount of help from other scientists. No one individual could possibly have devised a rational classification for some 60 elements by working alone, and in ignorance of the contributions made earlier by many other scientists. But certain historians of chemistry have gone a bit too far in allotting credit to others. Some of them have embraced the notion that one other chemist, Julius Lothar Meyer, made essentially the same discovery independently and at about the same time (1, 2). One of them has gone so far as to assert that Mendeleev and Meyer are "equally entitled to the discovery, and each acknowledged this" (emphasis added) (3). As we shall see, this statement is clearly contradicted by the historical record.
In a meticulous, book-length study of the pertinent documentary evidence van Spronsen finally comes to the conelusion that four contemporaries of Mendeleev, besides Meyer, should be recognized as "co-discoverers" of the system: A. B. de Chancourtois, J. A. R. Newlands, W. Odling, and G. D. Hinrichs (4, Sec. 5.11 and 16.8). This opinion has been repeated and reinforced by others (5). But if the concept of "co-discovery" is extended this far, by what criteria does one know where to stop?
The aim of the present article is to summarize the pertinent evidence and to evaluate the claims made either by, or in behalf of, some of the individuals named above. Based on this review, the writer's considered opinion is that Mendeleev's contribution was much superior to those of his contemporaries. The name of Henry G. J. Moseley has been paired with that of Mendeleev in the title because the former made the most significant improvement upon the Periodic Law, some 40...