Mendeleev on Periodicity: 1

The Periodic Law was announced by D I Mendeleev (1834 - 1907) at the first meeting of the Russian Chemical Society in March 1869, the paper from which the following is an extract being published in the Journal of the Russian Chemical Society, 1: 60-77 (1869). The Periodic Table was almost immediately reprinted in Zeitschrift fur Chemie, 12, 405 (1869); it appears also on Mendeleev's Memorial in St Petersburg. The driving force for Mendeleev's work was probably the writing of his famous 'Principles of Chemistry', published between 1868 and 1870. This appeared in three editions in English alone, as well as eight in Russian and some in French and German.

The famous predictions of then unknown elements were the subject of a second paper in 1871: see 'Mendeleev and Peridicity: 2'.

THE RELATION BETWEEN THE PROPERTIES AND ATOMIC WEIGHTS
OF THE ELEMENTS.

In undertaking to prepare a textbook called "Principles of Chemistry," I wished to establish somesort of system of simple bodies in which their distribution is not guided by chance, as might be thought instinctively, but by some sort of definite and exact principle. We previously saw that there was an almost complete absence of numerical relations for establishing a system of simple bodies, but in the end any system based on numbers which can be determined exactly will deserve preference over other Systems which do not have numerical support, since the former leave little room for arbitrary choices. The numerical data for simple bodies are limited at the present time. If for some of them the physical properties are determined with certainty, yet this applies only to a very small number of the elementary bodies. For example, such properties as optical, or even electrical or magnetic, ones, cannot in the end serve as a support for a system because one and the same body can show different values for these properties, depending on the state in which they occur. In this regard, it is enough to recall graphite and diamond, ordinary and red phosphorus, and oxygen and ozone. Not only do we not know the density in the vapour state for most of them, by which to determine the weight of the particles of the simple bodies, but this density is subject to alteration exactly like those polymeric alterations which have been noted for complex bodies. Oxygen and sulphur show this effect positively, but the relations between nitrogen, phosphorus, and arsenic offer further confirmation because these similar elements have particle weights of N₂, P₄, and As₄, unequal in the number of atoms among themselves. A number of the properties of the simple bodies must change with these polymeric changes. Thus we cannot be sure that for any element, even for platinum, there may not occur another state, and the location of an element in a system based on its physical properties would then be changed. Besides this, anyone understands that no matter how the properties of a simple body may change in the free state, something remains constant, and when the elements form compounds, this something has a material value and establishes the characteristics of the compounds which include the given element. In this respect, we know only one constant peculiar to an element, namely, the atomic weight. The size of the atomic weight, by the very essence of the matter, is a number which is not related to the state of division of the simple body but to the material part which is common to the simple body and all its compounds. The atomic weight belongs not to coal or the diamond, but to carbon. The property which Gerhardt and Cannizzaro determined as the atomic weight of the elements is based on such a firm and certain assumption that for most bodies, especially for those simple bodies whose heat capacity in the free state has been determined, there remains no doubt of the atomic weight, such as existed some years ago, when the atomic weights were so often confused with the equivalents and determined on the basis of varied and often contradictory ideas.

This is the reason I have chosen to base the system on the size of the atomic weights of the elements.

The first attempt which I made in this way was the following: I selected the bodies with the lowest atomic weights and arranged them in the order of the size of their atomic weights. This showed that there existed a period ih the properties of the simple bodies, and even in terms of their atomicity the elements followed each other in the order of arithmetic succession of the size of their atoms:

In the arrangement of elements with atoms greater than 100, we meet an entirely analogous continuous order:

Ag = 108; Cd = 112; Ur 116; Sn = 118; Sb = 122; Te = 128;

I = 127.

It has been shown that Li, Na, K, and Ag are related to each other, as are C, Si, Ti, Sn, or as are N, P, V, Sb, etc. This at once raises the question whether the properties of the elements are expressed by their atomic weights and whether a system can be based on them. An attempt at such a system follows.

In the assumed system, the atomic weight of the element, unique to it, serves as a basis for determining the place of the element. Comparison of the groups of simple bodies known up to now according to the weights of their atoms leads to the conclusion that the distribution of the elements according to their atomic weights does not disturb the natural similarities which exist between the elements but, on the contrary, shows them directly. .

All the comparisons which I have made in this direction lead me to conclude that the size of the atomic weight determines the nature of the elements, just as the weight of the molecules determines the properties and many of the reactions of complex bodies. If this conclusion is confirmed by further applications of this approach to the study of the elements, then we are near an epoch in understanding the existing differences and the reasons for the similarity of elementary bodies.

I think that the law established by me does not run counter to the general direction of natural science, and that until now it has not been demonstrated, although already there have been hints of it. Henceforth, it seems to me, there will be a new interest in determining atomic weights, in discovering new elementary bodies, and in finding new analogies between them.

I now present one of many possible systems of elements based on their atomic weights. It serves only as an attempt to express those results which can be obtained in this way. I myself see that this attempt is not final, but it seems to me that it clearly expresses the applicability of my assumptions to all combinations of elements whose atoms are known with certainty. In this I have also wished to establish a general system of the elements. Here is this attempt:

 Periodic Table according to D I Mendeleev, 1869

…… In conclusion, I consider it advisable to recapitulate the results of the above work.

1. Elements arranged according to the size of their atomic weights show clear periodic properties.

2. Elements which are similar in chemical function either have atomic weights which lie close together (like Pt, Ir, Os) or show a uniform increase in atomic weight (like K, Rb, Cs). The uniformity of such an increase in the different groups is taken from previous work. In such comparisons, however, the workers did not make use of the conclusions of Gerhardt, Regnault, Cannizzaro, and others who established the true value of the atomic weights of the elements.

3. Comparisons of the elements or their groups in terms of size of their atomic weights establish their so-called "atomicity" and, to some extent, differences in chemical character, a fact which is clearly evident in the group Li, Be, B, C, N, 0, F, and is repeated in the other groups.

4. The simple bodies which are most widely distributed in nature have small atomic weights, and all the elements which have small atomic weights are characterized by the specificity of their properties. They are therefore the typical elements. Hydrogen, as the lightest element, is in justice chosen as typical of itself.

5. The size of the atomic weight determines the character of the element, just as the size of the molecule determines the properties of the complex body, and so, when we study compounds, we should consider not only the properties and amounts of the elements, not only the reactions, but also the weight of the atoms. Thus, for example, compounds of S and Te, Cl and I, etc., although showing resemblances, also very clearly show differences.

6. We should still expect to discover many unknown simple bodies; for example, those similar to Al and Si, elements with atomic weights of 65 to 75.

7. Some analogies of the elements are discovered from the size of the weights of their atoms. Thus uranium is shown to be analogous to boron and aluminium, a fact which is also justified when their compounds are compared.

The purpose of my paper will be entirely attained if I succeed in turning the attention of investigators to the same relationships in the size of the atomic weights of non-similar elements, which have, as far as I know, been almost entirely neglected until now. Assuming that in problems of this nature lies the solution of one of the most important questions of our science, I myself, as my time will permit, will turn to a comparative study of lithium, beryllium, and boron.

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