How did Mendeleev’s first “Periodic Table” come about?

by Roberto Poeti

This post is also available in: Italiano (Italian)

The museum  

I think there cannot be such a strong emotion for a chemist as that one feels when visiting Mendeleev’s apartment at the University of St. Petersburg. He lived there from 1867 to 1890. The apartment is located on the ground floor of the Faculty of Chemistry and was connected to his laboratory. Since 1911, four years after his death, it has been transformed into a museum. I visited the Museum in 2009 on a trip to St. Petersburg. The paradoxical thing is that I had no idea of ​​the existence of this heritage. On the penultimate day of my stay in St. Petersburg, I realized, consulting the guide for the umpteenth time, that in a corner, in minute letters, the news of the existence of the museum was reported. So in the middle of August, a weekday in Russia, I was able to find the museum with difficulty and visit it. A visit all alone, with Dr. Natalia who was my guide without speaking English, but reading the information written in English from a notebook of her notes. I felt like I was visiting a small country museum without ostentation, all very informal. When you enter the apartment, after walking through a few rooms, you arrive at the study. Here time has stopped, everything has remained as it was when Mendeleev died. On one side of the study there is his desk, his armchair, the chess pieces. And it is precisely here where Mendeleev’s first “Periodic Table” come about

The study in Mendeleev’s apartment

Mendeleev’s study with the desk where he conceived the Periodic Table

The scientific context

Let’s first look at the context in which Mendeleev’s discovery fits. A few years earlier at the Karlsruhe congress (1861) a rigorous method for the determination of atomic weights had been defined by Cannizzaro. M. young chemist participates in the congress and is among those who is strongly impressed by the solution proposed by Cannizzaro. From that moment on, the atomic weights come out of the ambiguity that had previously characterized them and become one of the fundamental parameters of the identity of the elements. They are added to properties such as valence, specific weights, specific heats, atomic volumes, formation and nature of compounds. The latter have been thoroughly studied for years by M. The need to move from the specific to the general, that is, from the comparison of a few elements to a general classification scheme that includes all the elements, becomes increasingly urgent. In 1867 M. was entrusted with the important chair of Chemistry at the University of St. Petersburg, a position previously occupied by his mentor Voskresenskii. The reform of atomic weights and the discovery of new elements made all previous textbooks obsolete and brought out the need for their updating.

A new textbook

It is this need that M. feels so important and urgent. There is a need for a new textbook for the chemical training of Russian university students, ”I started writing [Principles] when I started lecturing on inorganic chemistry at the university after [the departure of] Voskresenskii and when, after having looked at all the books, I have not found anything to recommend to the students “. So he is about to write in 1868 the book “Principi di Chimica”. It is a work that deals extensively with General and Inorganic Chemistry. The first part of the work was completed at the end of 1868. There are treated the elements H, O, N, C, called organogenesis elements, in every detail. He has ordered the elements according to their value. Lastly, he follows the discussion of the group of halogens, considered strongly non-metallic. In mid-February 1869 he completed the first two chapters of the second part of the work by presenting alkali metals, specular to halogens, strongly metallic and both with valence one.

A problem arises

The problem that had already arisen in drawing up the preliminary plan of the second part of the work is now presented in all its acuity, namely which elements to treat after the alkali metals, but more generally which scheme to adopt in the treatment of all the known elements . He dealt with 8 elements, 55 remain (the known elements were 63). He believes it is necessary that the elements are arranged with a more rigorous criterion, such as the numerical one. It is a pedagogical need that determines the need to arrange the elements in a coherent system. The goal is essentially to prepare an updated and useful text for students.

A multifaceted person

With this purpose, February 17 begins the day by preparing for a long-planned ten-day trip that would take him to visit various dairy cooperatives in some districts of central Russia. M. is a multifaceted person, sensitive to social issues, he provides his voluntary collaboration to the Economic Society of St. Petersburg, an association we would call today a non-profit. In the first decade of the second half of the nineteenth century, Russia, under Tsar Alexander II Romanov, saw the implementation of important reforms such as the abolition of serfdom that emancipated forty million peasants, the reform of justice, education, of the press. All kinds of cooperatives are created, small artisan companies grow. M. who has always fought for the most advanced forms of economic development in his country was an ardent advocate of these new forms of production. He collaborates with the Economic Society which helps and stimulates these associative forms.

The first draft

And it is precisely on the morning of February 17 (according to the Juliano calendar) while having breakfast, before departure by train, that M. receives a letter from the secretary of the Association of Cooperatives informing him about the details of the trip. On the back of the letter, on which the imprint of the cup of the drink he was sipping remains, M. traces the first note of the day, the document in Fig.1. His day begins in an agitated way between the tight deadlines of a departure and the pressing thought on the book he is writing. What does the first draft of the day contain?

It is the first attempt to find a numerical criterion for arranging the elements. M.’s choice falls on the atomic weight. It should be noted that the atomic weight had a secondary place in the treatment of the elements present in the work that he was writing in this same period. The atomic weight was used as a secondary quality that showed the hierarchical order within a family of elements.

What do the numbers represent?

The first series of numbers is attributable to the atomic weight of the alkaline elements, the second series to a group of metals with valence two (the first position remains uncertain):? , Mg, Zn, Cd. The atomic weights of the two groups are arranged in ascending order. The novelty lies in the comparison between the atomic weights of pairs of elements of the two groups that correspond to each other. It is at this time of the morning that Mendeleev conceived the idea of ​​comparing chemically different elements in terms of the magnitude of their atomic weights. And this comparison accomplished this through the difference between their atomic weights. Mendeleev is working in an restless way, tailed by the commitments made, to the point that the arrangement of the elements becomes the priority. He will then postpone the journey by sinking into the new task. But from this first and confused attempt it is surprising that in the course of the day, through moments of stress and depression, as evidenced by his friend Inostrantzev who visited him that day, he achieves, as we shall see, the result represented by the compilation of the first table periodic.

A second attempt

From this first attempt, comparing the alkali metals with the group of other metals, M. did not obtain as a result a scheme that convinces him. However, he insists on the criterion adopted, namely the difference between atomic weights. In another scattered note that he left us and is supposed to follow the previous one, he orders this time in a table the groups of non-metals, with increasing value, best known at that moment.

The main criterion that follows in composing this table is always constituted by the comparison between the atomic weights of the elements of adjacent groups. In all of these cases, except one (Tellurium), the atomic weight of successive elements of different groups in a column decreases by approximately the same amount, a few atomic units. It follows that the position of an element is correct if the size of its atomic weight with respect to the different elements, adjacent in the same column, is a few units. This criterion will acquire more and more, as it progresses in the construction of its system, a dominant prominence. In this regard in the report “The correlation of the properties and atomic weights of elements” which he writes for the Russian Chemical Society (March 6, 1869), shortly after the compilation of his first Periodic Table , will make explicit the choice of this criterion: “The purpose of my essay would be entirely achieved if I succeeded in attracting the attention of scholars to the relationships between the dimensions of the atomic weights of non-similar elements [of different groups], which, it seems to me , have until now been ignored “.

One step forward

In a subsequent draft, which always shows the date of February 17, M. inserts other elements in Tab 1, going from fifteen to forty approximately. The draft consists of two distinct attempts to order the elements, arranged on the same sheet. The first, in chronological order, is the one in the upper part and the second following in chronological order is placed in the lower part. Between the first and the second M. makes significant changes.

Three groups are added with respect to Table 1, that of alkali metals, alkaline earth metals and the group of metals that we have already encountered in the first attempt made by M. in the first draft Mg, Zn, Cd. There are other elements that M. clearly has difficulty in inserting and that he places in the margins. The interesting thing is the change that occurs by passing from the arrangement at the top to the next one at the bottom:

The central body of the Periodic Table takes shape, the third group is missing, the position of Beryllium and Hydrogen are uncertain.

An incessant work

M.’s work continues without pause. The document that most of all expresses the intellectual effort of M. is the following , Fig.3, subsequent in time to that of Fig 2.

The draft of Fig. 3 is a real experimental laboratory where the elements are placed singly or in groups, moved, deleted, replaced. The real novelty with respect to the draft of Fig 2 lies in having expanded the table, managing to place the transition metals that had represented its biggest obstacle both for the uncertainty of some atomic weights, and for the lesser knowledge of their properties. . Find a location for Be, giving it the correct atomic weight, overcoming the previous difficulties. Identify for the first time the third group with B – Al. It inserts 61 elements into the system, almost all of those known. At the top of the scheme are the rare earths to which incorrect atomic weights are attributed, and whose insertions in the Periodic Table it will always be a problem for M. Uranium is attributed half of its atomic weight. At the end, a system is outlined with atomic weights that grow by a few units from the bottom upwards and an arrangement of similar elements by groups, placed in the same horizontal row, while the elements whose atomic weights vary gradually are lined up in periods. The overall result in the end is represented by a sufficiently compact and coherent system.

The empty spaces

The “revolutionary” aspect of this work is the presence of different empty spaces between some elements belonging to different groups. Between these last elements there is a difference between their atomic weights that is too high, not of a few units (remember that this is the fundamental criterion used by M.). The possible existence of elements yet to be discovered that fill these holidays is still a premature hypothesis. M. tried to build a didactic tool, the empty spaces have the meaning that the scheme is not definitive both because different atomic weights are provisional, not correct or not known, and because the empty spaces could be reduced with a better distribution of the elements.

Defects are providential

They are therefore a sign that the system still needs to be improved. It is surprising how these gaps that arise as defects in the system will then become the feature that will legitimize the system itself in the eyes of the world. In no arrangement of the elements made in that same period, that of John Newlands (1865), Lothar Meyer (1869) and De Shancourt, was expected the presence of voids in their schemes. The criterion that M. adopts has a unique peculiarity, it shows the points where it is to be corrected. Despite the corrections, the improvements that M. will bring to his first scheme, the gaps will continue to exist. M. will begin to think about the possibility that they are the expression of missing, undiscovered elements. But upon compiling his first Periodic Table this thought has not yet matured.

The table overturns

At the end of a long day of intense work, after a short rest, Mendeleev makes one last change to the previously seen scheme. Let’s imagine that we rotate the diagram in Fig. 3 by 180 ° around its X axis. We get the diagram in Fig 4 where the atomic weight of the elements grows from top to bottom (as in today’s Periodic Table). The first version reflected the criterion adopted by M., in fact he made the difference between the dimensions of the atomic weights. In the difference between two numbers, the largest is positioned at the top, hence the first arrangement of the elements.

It is in this last form that M. arranges the elements in his latest draft (Fig 5), which he will print immediately afterwards. He makes changes. He fills the empty space between Fe and Cu with Ni and Co to which he nevertheless assigns the same atomic weight (the position of both will be resolved with knowledge of the atomic structure). Again he appears uncertain in placing Mg. He will insert the Be, Mg, Zn, Cd group again (it was the group that he had initially compared with the alkali metals), separating Mg from the second group (Ca, Sr, Ba)

A familiar periodic table

Perhaps looking at this latest draft, we have the impression that it is very different from the version we are familiar with today. The element groups are arranged in rows, while the periods in columns. But if we separate the draft into three parts (Fig 6) and then reposition them in a different order (separating the rare earth elements) but respecting the progression of the atomic weights, we get a shape that is familiar to us (right figure). In modern terms in the first Periodic Table  of M. we find distinct the elements divided into the three blocks s, p, d, with respect to the filling of the orbitals, as in today’s version of the Peridic Table

Thus was born towards the evening of February 17, 1869, after a long day of intense and stressful intellectual commitment, in which moments of exaltation alternate with periods of depression, the first arrangement of Mendeleev’s elements. He has the draft printed, 150 copies in Russian and 50 in French, which he sends to various chemists. He calls it “Attempt”. M. is well aware of the still provisional nature of his work and of the uncertain positions of various elements. But the foundations are laid. At this moment the awareness of a periodic law underlying his scheme has not yet matured. In fact it is unlikely that Mendeleev understood the generality of his system when he developed it in February 1869. Had he been aware of the implications of the periodic system, he most likely would not have relegated to his friend Nokolay Menshutkin the initial presentation of it to the Russian Chemical Society in March 1869 while inspecting cooperatives on behalf of the St. Petersburg Economic Society. Just as he was still far from the possibility of foreseeing new elements that would occupy the empty spaces.

The first article

He will write the first scientific article on his first Periodic table in April 1869, two months after he had the first draft printed. At the end of the article, he makes a list of eight advantages that his system would have over other competing classifications in that period. At the sixth point, the first hint of the possibility of foreseeing new elements appears. It is a weak prediction, it places it only at the sixth point and it is very vague. From his notes prevails the choice of trying to fill the empty spaces with already existing elements based on chemical properties, and see if, for example, their atomic weights had been incorrectly measured. Only in August 1869 in an article on atomic volume he will abandon this approach and some of his previous vagueness. Over the next two years, he will make several changes to his first draft, providing various versions (but that’s another story).

A reconstruction of the T.P. done a posteriori

When we imagine constructing the Periodic Table placing the elements according to the order of their increasing atomic weights and observing that elements with similar properties reappear at certain intervals, we are making an a posteriori reconstruction that is effective in teaching practice, but which is not correct. as a historical reconstruction. The periodic properties and the law of periodicity emerged later, some time after the compilation of the first Periodic TableThe latter already contains the characteristics of periodicity, but the recognition of a law of nature was more complex than the presence of a certain regularity in the system. In reality, as we have seen, Mendeleev does not use the atomic weight but rather the difference between the atomic weights as a criterion for creating a system of elements. His first Periodic Table shows how the groups of elements have been superimposed, like the elements of a Voltaic stack, according to the criterion of the minimum difference between the atomic weights of adjacent elements of different groups. And where the groups of elements were not well characterized and differentiated, as among the metals of the transition series, he managed to find, after a tormented work made of trial and error as we have seen, their definition and placement following the main criterion adopted by him. The periodicity is not discovered and therefore immediately recognized, but rather emerges as a consequence of the criterion he adopts in his system. It is a consequence of this, hence the longer time to recognize its importance.

A paradox

The periodic table and the enunciation of the law of periodicity that will be the final goal of M. have constituted the most powerful tool in the hands of chemists and physicists to get to understand the structure of the atom. And it is paradoxical, but in the history of science there is no lack of other examples, that M. who built this instrument, had a good dose of mistrust towards the atomic theory and subsequently faced with the possibility that the atom had a structure subatomic was skeptical

Images of the Museum

In my blog there is an article on Mendeleev’s apartment – museum at the University of St. Petersburg. There are pictures of the rooms in the apartment, photos and paintings hanging on the walls, and various documents collected on the bulletin boards. The captions have been translated from Russian. The article is in the following two formats:

The D.I.Mendeleev Home Museum in St. Petersburg

This post is also available in: Italiano (Italian)

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