V. N. Kondrat’ev. Physical and Chemical Properties of Molecules.
Series “The Newest Trends of Scientific Thought,” No. 15, State Publishing House, Moscow—Leningrad, 1928, 103 pp., price 1 ruble 25 kopecks, hard binding 15 kopecks.

The history of physics and chemistry shows that the development of these sciences proceeds by leaps, if one may put it that way—in distinctive “quanta.” From time to time a major experimental discovery or brilliant theory appears, which in a very short period makes it possible to clarify and unify a multitude of facts that had previously remained scattered and poorly understood; and, what is very important, such a discovery or theory directs new experiments that greatly broaden our horizons in the given field.

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Let us recall, if only from the field of thermodynamics, the establishment of the second principle by Clausius, then the principles of equilibrium by Gibbs, and finally the heat theorem of Nernst; from other fields let us recall the discoveries of Fresnel, the theory of Maxwell, the discovery of radioactivity, the quantum theory of Planck, and so on. Each time, after such a discovery, following a certain incubation period, there begins a period of vigorous development of the corresponding field of science; and since it is characteristic of us to judge historical development by the tangent to the curve of the rate of development at a given moment, it seems not only to outside observers but also to adepts of the given field of science that this time a radical means has been found which will allow us, in a short time, to resolve the age-old questions completely. But years pass, and each time it turns out that the theory begins first to encounter mere difficulties, and then insurmountable obstacles; the rate of development of the given field begins to fall, its adherents stop, look back, and then there begins a period of critical attitude toward the given theory. In this period all enthusiasms are cast aside, the limits of application of the theory are delineated, and the latter, in a purified, often stripped-down form, takes its place among many of its predecessors. Then follows a long period of quiet, peaceful, and fruitful use of this theory. Such a critical period has already been passed for many of the discoveries and theories indicated above: let us recall the criticism of the first two principles of thermodynamics (Duhem and others), the criticism of the doctrine of Gibbs (van der Waals and others), and so forth.

One of the most recent in time among the ingenious theories was Bohr’s theory of the structure of the atom. The first period of its development (not counting the incubation period) passed before our eyes. We all well remember the grandiose hopes that were placed upon it: it seemed to many that the Bohr model of the atom was not a scheme but a reflection of real relations in nature, and that, by virtue of this, the problems of chemistry were in principle solved, and that, in the words of one physicist, only routine work remained for chemistry. A model of enthusiastic attitude toward this theory may be the small book by O. Chwolson, The Spectrum of X-Rays and Bohr’s Theory, 1920. Now we know that the difficulties encountered by the theory have turned into insurmountable obstacles; it has become necessary to create new theories. There is no doubt that criticism of Bohr’s theory is at times going too far, and one somehow feels sad when one reads the following words of Sommerfeld: “The elementary representation of the temporal motion of the atomic model cannot be preserved in the new theory. The marvelous visual clarity of the microscopic planetary system, according to the present state of quantum theory, has apparently perished irrevocably.” Sommerfeld consoles us with the fact that in the new theory (Schrödinger’s) there is something “more important than visual clarity—mathematical simplicity.” Unfortunately, this is a kind of simplicity in the presence of which the theoretical specialists themselves declare that they do not know how to translate it from the mathematical language into the physical language. At the present time, however, we cannot do without

theory of Bohr, even in its simplest form; however harsh the criticism of it by theoreticians may be, the experimenter must resort to it both in his works and in attempts to present this field of science. True, we notice a certain haste on the authors’ part in using this theory; they hasten rather to pass over to a language that does not recall the atom model. On the other hand, we also note a somewhat softened attitude of physicists toward static theories of the structure of the atom.

The thoughts expressed here were prompted by V. N. Kondrat’ev’s excellent small book, Physical and Chemical Properties of Molecules. After a brief and successful—not so much exposition as use—of Bohr’s theory, the author turns to the extremely interesting and important question of molecular spectra and the energy levels of molecules—an area that at present is the battlefront of physics. A bright ray of light has been cast into this exceedingly complex and confused chapter of physics, from whose further development we may expect much. In the exposition of these chapters of his book the author has shown an ability to present difficult problems admirably, and it is a great pity that he did not dwell on these questions somewhat more fully; it is vexing if external forces (the publishing house) limited the author’s scope. The author treats just as concisely the subsequent chapters on the structure and formation of molecules and on their dissociation.

V. N. Kondrat’ev’s book vividly shows the enormous difficulties that physics encounters in interpreting molecular phenomena even in the simplest cases, and shows no less vividly that in the physical interpretation of phenomena we cannot do without a model, even an imperfect one. Perhaps it would not be a great misfortune if physicists used, somewhat more boldly, even if only as working hypotheses, models of molecular structure. When two atoms, each with its own suite of electrons, combine into a molecule, one or several of their electrons become “common” electrons characterizing the molecule. What does a common electron mean? If such an electron revolves around both nuclei, then what becomes of its quantum numbers, for the most part so sharply different for the valence electrons of two different atoms? Apparently this question will be illuminated by the study of molecular spectra; this is indicated by the analogy between molecules and atoms in the region of molecular multiplets. The presentation of this question in V. N. Kondrat’ev’s book is one of the most interesting passages in the book.

Unfortunately this question is treated too briefly; just as briefly treated is another capital question, that of the connection between maximum valence and the multiplicity of atomic terms. And yet it is precisely here that a number of questions at once arise for the chemist. According to the author, “the maximum valence of a given atom (equal to the number of unpaired electrons) \(V\) is one less than its maximum multiplicity \(M\).” The highest multiplicity of oxygen is equal to 3, “therefore the valence of oxygen must not exceed 2.” But what is to be done with those cases in which oxygen is tetravalent? I am speaking of the numerous oxo-

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compounds, and finally of interesting basic oxygen-containing compounds of beryllium, zinc, and zirconium. For one of the latter, Morgan and Bragg showed that its molecule is a regular tetrahedron, at the center of which there is oxygen, and at the vertices—four beryllium atoms.

This rule is magnificently confirmed, for example, in the elements from potassium to iron, but it fails for cobalt, nickel, and copper. And the group of rare-earth elements? There, for 15 consecutive elements the principal valence—indeed in many cases the maximum valence—is equal to 3; in rare cases (cerium, praseodymium, and terbium) it rises to 4. Meanwhile the multiplicity, quite correctly with the change in ordinal number, passes through a maximum: it is equal to 4 for lanthanum, reaches 11 for gadolinium, and then falls again to 4 for lutetium.

It may be that it would have been more cautious to seek a connection between multiplicity and the number of outer mobile electrons lying outside already filled shells, as Hund does, and not between multiplicity and valence (a chemical concept). For the very connection between this number and valence is still far from clarified for very many elements past which the chemist cannot pass. It would also have been highly desirable to hear the opinion of a competent person on the relation between optical and valence electrons: for in a number of cases optical electrons reveal themselves at temperatures at which the element no longer possesses valence.

Unfortunately, because of the concision of his exposition, V. N. Kondrat’ev was unable to dwell on these questions, important for the chemist.

After the atom, physics began to concern itself with the molecule; undoubtedly, this time it is more cautious—perhaps more cautious than it ought to be. Incidentally, it gains from this caution: there is less disappointment both in the present and in the future. It would not hurt, however, to strengthen caution in a somewhat different direction, namely the chemical one. When a physicist working in this field says that the secret of chemical forces has been uncovered, that the current of chemical thought is directed along a physical channel, when he speaks of electronic chemistry and the chemical properties of molecules, there is a fair dose of enthusiasm here. One ought to think about what explains the fact that precisely in the period of the flourishing of the dynamical theory of the atom, such leaders of chemistry as Lewis and Langmuir were developing their, admittedly awkward, static theories. One cannot suppose that Lewis and Langmuir were not in a position to understand and sort out Bohr’s theory; evidently there was another reason. This reason consists in the fact that the theory of the atom has until now been in a position tangent to chemistry, or at best in the position of a secant, very close to the boundaries, but not to the center of the domain of chemistry. Physics has lit a large, interesting, and promising bonfire at the threshold of chemistry, and only at the threshold; chemists, however, have to work in the twentieth room from the threshold, and many, very many of them do not even see this bonfire. Attempts

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Lewis and Langmuir’s transition from imagery and accounting to symbolism (Sedgwick) is a desire to transfer the primary ideas of the structure of the atom as quickly as possible into the depths of chemistry.

It seems to me that physicists ought to be more modest and cautious when they speak about chemistry; in any case, it would be highly desirable to adhere to the rule that the title of a book should be equal to, or narrower than, the contents of the book; otherwise, after reading the book, there remains a certain dose of disappointment that prevents one from duly appreciating it.

In conclusion, I shall allow myself to express the hope that so great an expert as V. N. Kondrat’ev, who is able to expound complex problems so well, will write a more extensive book on these questions, in which he will speak not only of the successes of the theory, but also of the difficulties it encounters, however numerous they may be. A book that speaks only of successes reminds me, in part, of a tombstone slab; a book that also speaks of difficulties is a call to think and to work toward the further solution of the problems that lie ahead.

A. Rakovsky.