The Periodic Table
I just returned home from being interviewed for a new public television program on the mystery of matter and the search for the elements. It was very gratifying to see how keen the film-makers were on understanding precisely how Mendeleev arrived at his famous first periodic table of 1869. This in turn meant that I had to thoroughly review the literature on this particular historical episode, which will form the basis of this blog.
The usual version of how Mendeleev arrived at his discovery goes something like this. While in the process of writing his textbook, The Principles of Chemistry, Mendeleev completed the book by dealing with only eight of the then known 63 elements. He ended the book with the halogens, including chlorine, bromine and iodine. On moving on to the second volume he realized that he needed an organizing principle for all the remaining elements. Before arriving at any new ordering principle he started volume 2 by discussing another well-known group of elements, the alkali metals that include lithium, sodium and potassium.
Mendeleev then wondered what elements should be mentioned next and toyed with the idea of turning either to the alkaline earth metals like calcium, barium and strontium or perhaps some intermediate elements including zinc and cadmium which share some but not all the properties of the alkaline earths. Another possibility which he contemplated was a group containing copper and silver which show variable valences of +1 or +2 and so could represent a stepping stone between the alkali metals and the alkaline earths which display oxidation states of +1 and +2 respectively.
Then on 17 February 1869, Mendeleev’s world virtually stood still and it continued to do so for a further 2 or three days during which he essentially arrived at his version of the periodic table and the one that had the greatest impact on the scientific community. It is generally agreed that this was the discovery of the periodic table, although at least five other versions had been previously published, albeit rater tentatively.
On the 17th of February Mendeleev decided against going on a consultancy visit to a local cheese co-operative in order to stay at home to work on his book. It appears that at some point in the morning he took the invitation to the cheese co-operative and turned it over in order to sketch some ideas about what elements to treat next in his book (figure 1). This document still exists in the Mendeleev Museum in St. Petersburg and it is frequently brought out of the coffers for visiting documentary film-makers wanting to capture Mendeleev’s crucial moment of discovery.
The sketched symbols suggest that Mendeleev’s first attempted to compare the alkali metals with the intermediate group containing zinc and cadmium. He calculated differences between pairs of elements belonging to each of these groups in the hope of finding some significant pattern. But he appears to have been disappointed because the differences between the corresponding elements he considered show no regular pattern.
Nevertheless, Mendeleevdid not quite dismiss the idea of following the alkali metals by the group containing zinc and cadmium because this is precisely what he did in a second classic document in which he now included many more known groups of elements in the first of two tables of elements which appear on the same sheet of paper (figures 2 and 3).
Whereas the upper table shows the zinc and cadmium group directly above the alkali metals, the lower of the two tables shows a rearrangement in which Mendeleev has decided to place the typical alkaline earth metals next to the alkali metals by moving the alkali metals up the table as an entire block. The net result is that the halogens are followed by the alkali metals, which in turn are followed by the alkali earths. The consequence of this move is that the sequence of atomic weights now appears more orderly than it did in the earlier upper table on the same page. As a result of this simple change Mendeleev appears to have realized that a successful periodic table requires not only a correct grouping of elements in adjacent rows but also a set of smoothly increasing sequences of atomic weights.
Here then is Mendeleev’s ‘aha’ or ‘eureka moment’. Here is where he first sees that the periodic table is a display of chemical periodicity that is itself a function of the variation of atomic weight. For example, note the atomic weight sequence of Cl (35.5), K (39) and Ca (40) in the lower table as compared with the less pleasing, although still increasing sequence of S (32), Cl (35.5), Ca (40), in the upper table that he had arrived at earlier in the day. Alternatively, consider the placement of K (39) which seems out of place next to Cu (63) in the upper table as compared to its proximity with elements of similar atomic weights in the lower table.
The essential point seems to be that Mendeleev began by considered groups of chemically similar elements and that the notion of ordering according to atomic weight came to him later. And this document appears to be precisely where he arrived at this conclusion.
Interestingly, the current director of the Mendeleev Museum, Professor Igor Dimitriev, disagrees with this account of the development. He believes that the document sketched on the back of the invitation from the cheese co-operative (figure 1), did not precede the two-tables on a single sheet document (Figures 2 and 3). He does not believe that the document shown in figure 1 had such an influence of the development in Mendeleev’s thought process as has generally been supposed.
Dimitriev’s objections are based on his proposal that groups of elements were not widely recognized at this time and that it was rather the sequence of atomic number values that led the way for Mendeleev in the course of his discovery of the periodic table. But this may be a little short-sighted in my view, because if one looks further afield at the earlier evolution of the periodic table among other chemists, working in other countries, one finds that groups of elements had been well recognized for a long time prior to Mendeleev’s work. This includes the work of Döbereiner, Gmelin, Lenssen, Pettenkoffer, De Chancourtois, Newlands, Odling, Hinrichs, and Lothar Meyer just to mention a few relevant names.
There is little doubt in my own mind that the notion of groups of chemically similar elements was well rather established and that it would have been natural for Mendeleev, who followed the above named authors, to begin with this notion. On the other hand the idea of using the sequence of increasing atomic weights to order the elements was nowhere near as well-established and it had only been a few years since the Karlsruhe conference of 1860 at which atomic weights had been unified and rationalized to produce a more or less definitive list of values that every chemist agreed with.
But you the reader may now be thinking, “but surely Dimitriev knows all of this?”. I think the answer to this question is both yes and no. I suspect that his custodianship of the St. Petersburg museum and archives may have led Dimitriev to concentrate upon the work of Mendeleev above that of all others. Finally, could it be that Dimitriev, who like Mendeleev is a Russian, may have allowed national pride to influenced his judgment of the issue and to perhaps downplay the contributions of foreign scientists.
But let’s return to Mendeleev’s discovery. What did he do after he had produced the lower table in figure 2? The popular story is that he then set about playing a game of chemical solitaire or ‘patience’ using a set of cards that he had carefully made-up to include the symbols for all the known 63 elements and their atomic weights. The aim of this well-known game is to arrange the cards in two senses. First of all the cards must be in separate suits and secondly they must be in order of decreasing values starting with king, queen, knave, ten and so on reading from left to right. Unfortunately no such set of cards has ever been found among Mendeleev’s belongings which raises the question as to whether the story may be merely apocryphal. (The plot thickens further when one learns that Mendeleev kept almost everything as soon as he realized that he would become famous. No such cards have ever been found, although it could just be that Mendeleev had not quite realized his impending fame at this stage.)
But I don’t think it really matters whether the story of the cards is actually true or not. The game of chemical solitaire provides such a good analogy that it is more important to focus on that than trying to determine whether Mendeleev actually used this approach or not. In the case of the periodic table, there is a beautiful analogy given that the elements are arranged in groups as opposed to suits, and along another direction they are arranged in order of increasing values of atomic weights, as opposed to decreasing values on cards.
Although Mendeleev was a true genius for discovering the periodic table, there is a real sense in which the periodic system is inevitable and provided by Nature itself. It was just a matter of uncovering this profound truth. What I am trying to get at is that Mendeleev did not have any choice in how to arrange the elements. At the end of the day they had to be arranged in the same way as a deck of playing cards must be arranged in the game of patience. There is no two ways about it. When the game is completed everyone can see it.
It is the same with the arrangement of the elements. Although the pattern could only be dimly seen at the beginning this was partly because of inaccurate values of atomic weights and because the correct ordering principle had not yet been recognized. Once it was recognized the game was virtually over and it became a matter of filling-in the remaining details. Of course these details were not quite as trivial as I may be implying. They included an entire group of missing elements that neither Mendeleev nor anyone else had predicted — the noble gases. They included the discovery of several missing elements, many of whose properties Mendeleev succeeded in predicting rather well. They also included the vexing fact that atomic weight doesn’t provide the optimal ordering principle.
If atomic weight ordering is followed strictly as many as four pairs of elements occur in reversed positions. In order to clear up this further issue it had to wait until the discovery of atomic number in 1913 and 1914 but that will be the topic of a future blog. The broad outline of chemical solitaire was worked out by Mendeleev above all other contributors and it was first glimpsed on that famous day of 17 February 1869. (This is the date according to the older Julian calendar that was used in Russia at this time. It differs from the more recently developed Gregorian calendar that was introduced to many other western countries in 1582. In 1869 the difference between the two calendars amounted to 12 days.)
Figures 1 & 2 from Mendeleev’s two incomplete tables of February 17, 1869.
Figure 1 source: Igor S. Dmitriev, “Scientific discovery in statu nascendi: The case of Dmitrii Mendeleev’s Periodic Law,” Historical Studies in the Physical and Biological Sciences, Vol. 34, No. 2, 2004.
Figure 2 source: B. M. Kedrov and D. N. Trifonov, Zakon periodichnosti…, Moscow: Izdatel’stvo “Nauka,” 1969 (via Heinz Cassebaum and George B. Kauffman, “The Periodic System of the Chemical Elements: The Search for Its Discoverer,” Isis, Vol. 62, No. 3, 1971).
Figure 3 Smith, J. R. (1975) ‘Persistence and Periodicity’, unpublished PhD thesis, University of London. Source: Eric Scerri.
Headling image credit: The periodic table of the elements. Public domain via Wikimedia Commons.