Before quantum chemistry: Erich Hückel andthe physics-chemistry interface

H K*

1. Introduction

In the history of chemistry and its interface with physics, Erich Hückel’sname is firmly associated with two major theories, the 1923 Debye-Hückeltheory of strong electrolytes and his 1931 quantum-mechanical theory of thestructure of the benzene molecule. Hückel was not a prolific author, but thesetwo contributions alone made him one of the pioneers and central figures ofmodern theoretical chemistry. In this paper I deal exclusively with his workbefore the advent of quantum mechanics, that is, Hückel’s contributions untilthe age of thirty. Among his Jugendarbeiten, the Debye-Hückel theory wasby far the most important, and for this reason I discuss this work in somedetail. I also consider other of Hückel’s early works and, in general, portraya young physicist’s career and move from physics to theoretical chemistry inthe 1920s.

A historical analysis of the career and work of the young Hückel is notonly interesting from a biographical point of view, but also, and perhaps evenmore so, from the more general perspective of illuminating the relationshipbetween physics and chemistry in Germany during the Weimar period. Thistopic has been discussed in relation to the emergence of quantum chemistry

* History of Science Department, University of Aarhus, Ny Munkegade, 8000 Aarhus,Denmark. This paper is based on a lecture I gave at the Hückel commemoration col-loquium at Phillips-Universität Marburg in October 1996.

C 2001: V. 43: . 1–16C Munksgaard 2001. Centaurus ISSN 0008-8994. Printed in Denmark. All rights reserved.

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(Gavroglu and Simoes 1994; Karachalios 2000), whereas the relationship be-fore quantum mechanics has received but scant historical attention. Through-out Hückel’s scientific career, he spanned both of the two sister sciences,physics and chemistry. By studying his work in its historical context we mayuse it as a mirror from which we can get some insight in the way physicsand chemistry interacted during the period; and, not least, how they did notinteract.

2. Education and early career

Erich Hückel, born 1896, and his two brothers, Walter and Rudi, were raisedunder the strong influence of their father, Armand Hückel, a retired medicaldoctor and science enthusiast (Hückel 1975; Hartmann and Longuet-Higgins1982). The father introduced his sons to the world of science at an early age.The paternal introduction included experiments in the laboratory set up inthe family house in Göttingen and also the study of books in physics andchemistry, including Wilhelm Ostwald’s Die Schule der Chemie. Whereas theone year older Walter decided to study chemistry, Erich entered in 1914Göttingen University to study physics, unfortunately shortly before the out-break of the First World War. The war interrupted the studies and careers ofmany young scientists, Hückel included. On the other hand, he was fortunatein not being enlisted as a soldier and sent to the front. Instead he was as-signed to work as an assistant to the aerodynamicist Ludwig Prandtl in hisaeronautics laboratory in Göttingen, the later Aerodynamische Versuchsan-stalt. Later, in 1918, he served at a military station in Warnemünde where hehelped in scientific work related to the German navy’s air force (Trischler1992). It was here that he made his debut in science with a series of aerody-namical investigations on aeroplane profiles, published in the Technische Be-richte der Flugzeugmeisterei.

After the war Hückel returned to Göttingen to resume his studies of physics.In 1921, he married Anne Zsigmondy, a daughter of the Göttingen chemistRichard Zsigmondy who four years later would receive the Nobel Prize for hispioneering work in colloid chemistry. The same year Hückel wrote his disser-tation under Peter Debye on the scattering of X-rays by what at the time wasthought to be liquid crystals.1 This was Hückel’s first and only experimentalwork. His future lay within theoretical physics and chemistry and he soon

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emerged as one of the mathematically gifted physicists in which Göttingen wasso rich. For a brief period he served as assistant for the mathematician DavidHilbert, a sure sign of an unusual mathematical talent. Yet Hückel seems tohave been uncertain about his mastery of mathematics in the highly competi-tive Göttingen environment, where self-confident Wunderkinder like WolfgangPauli and Pascual Jordan set new standards. He later modestly remarked thateven if he had remained in Göttingen in the golden years of 1925–26, his talentof mathematics would probably have been insufficient for him to contribute tothe formation of quantum mechanics (Hückel 1972a, p. 3).

Physics in Göttingen was dominated by the two professors, Debye andMax Born, who both exerted a strong influence on young Hückel, althoughin rather different ways. Hückel was still an apprentice dependent on hisprofessors. His first important works were suggested by, and done in collabor-ation with, Born and Debye, respectively. The young physicist collaboratedas a junior partner and perhaps more as a calculator than the creator ofphysical ideas. Debye, who was director of the Göttingen physics institute,moved in 1921 to Zurich’s polytechnical institute, the ETH, and was replacedby Born as head of the department of mathematical physics. AlthoughDebye’s reputation rested on both his experimental and theoretical research,he was primarily considered a theoretician. Born, who was trained as a puremathematician, had a strongly mathematical approach to physical problemsand was rather foreign to experimental research. Thus, under the influenceof Debye and Born, Hückel was brought up in an environment that valuedmathematical expertise highly and in which mathematical aptitude was con-sidered a necessity for a career in physics.

3. Molecular spectra and Formelrechnerei

When Hückel came under Born’s wings in 1922, he entered among a seriesof assistants, which was truly remarkable. Before Hückel, Born’s assistantsincluded Pauli, Heisenberg and Jordan, and he was succeeded by FriedrichHund, Lothar Nordheim, Walter Heitler and Leon Rosenfeld – all comingleaders of theoretical physics. Incidentally, according to Born’s autobio-graphy, Hückel ‘‘graduated in 1923, and some years later he became my assis-tant,’’ which is obviously an error (Born 1978, p. 214). It illustrates howcautious one should be in using information from scientists’ recollections.

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As Born’s assistant, Hückel was assigned his first major theoretical work,namely, to assist the professor in developing a quantum theory of moleculesconsisting of two or more atoms. At that time the Bohr-Sommerfeld quantumtheory had been successfully applied to some atomic spectra, but molecules andtheir spectra had only recently been investigated and seemed more resistant tothe methods of quantum theory. The problem was one of the Göttingen insti-tute’s central research programmes. For example, it was examined in great de-tail by Pauli in his dissertation, a quantum-theoretical investigation of the sim-plest possible molecule, the H2

π ion. Pauli’s involved calculations were unableto reproduce the correct ionisation energy and the failure indicated thatexisting quantum theory might have its limitations when it came to molecularstructure (Jensen 1984). The theory of the band spectra of diatomic moleculeshad been treated by Adolf Kratzer, a Munich physicist who had spent a year inGöttingen as Hückel’s predecessor as assistant to Hilbert. In the theory of Bornand Hückel, essentially an extension of Kratzer’s work, the atomic centres wereassumed to oscillate with small amplitudes. By expanding the Hamiltonian interms of the parameter associated with the amplitude, they found an expressionfor the energy of polyatomic molecules (Born and Hückel 1923).

It is worth noting that the theory of Born and Hückel, and also the slightlylater Debye-Hückel theory, appeared in the Physikalische Zeitschrift ratherthan in the more prestigious Zeitschrift der Physik, which at the time emergedas the world’s leading journal in theoretical physics. One of the reasons wasundoubtedly that Physikalische Zeitschrift was edited by Born and Debye,with Hückel serving as the redactory editor. In this way, the articles weresecured quick and easy publication.

The Born-Hückel theory was highly theoretical and the energy formulae,especially for molecules consisting of more than two atoms, so complicatedthat they had almost no empirical relevance. This was a feature typical ofBorn’s mathematical approach to physics which emphasised detailed and in-volved calculations rather than physical insight. Indeed, Hückel seems to havebeen brought in as a mere calculator, a work he did dutifully and com-petently, but without enjoying it. In 1970 he recalled that his work consistedin ‘‘rather distasteful Formelrechnerei, i.e., plugging-in of numbers into for-mulas and calculating the answers,’’ a kind of work he ‘‘enjoyed but little’’(Hückel 1972a, p. 3). Whereas Hückel soon left molecular calculations, Born,this time with Heisenberg, developed a more extensive and systematic versionof the Born-Hückel theory (Born and Heisenberg 1924). The Born-Heisen-

Before quantum chemistry 5

berg theory was an orgy of calculations and even less suited for practicalapplications than the work with Hückel.

The work with Born was not only Hückel’s debut in theoretical physics, butalso his first research in what later became chemical physics and thus his firstresearch related to chemistry. Hückel never received any formal training inchemistry and his knowledge of the field was restricted to scattered reading ofhis own, discussions with his brother, and whatever experiments he had madein his father’s laboratory. This may seem an unusual and wholly inadequatebackground for entering chemistry, but it was a background shared by severalother physicists dealing with areas of chemical theory. Lack of knowledge oftraditional chemistry was not unusual among the new generation of physicistswho moved into theoretical chemistry in the 1920s and helped establish the fieldof chemical physics. For example, Friedrich Hund, the Göttingen physicist andpioneer of quantum chemistry, was largely ignorant about chemistry when heentered the field. In an interview of 1963, he told that he was ‘‘somewhat embar-rassed because I only knew very little chemistry. I have never studied chemistryand felt therefore uncertain about all chemical questions.’’2

Molecular spectra was a meeting point for physicists and theoretically in-clined chemists and the central area of quantum chemistry before quantummechanics entered the scene. The tradition was developed both in Germanyand the United States, but in widely different ways and with different degreesof success. In America, chemical or molecular physics became a truly inter-disciplinary field investigated in a pragmatic manner by researchers who wereas much experimentalists as theoreticians, and as much chemists as physicists(Assmus 1993; Gavroglu and Simoes 1994). Scientists such as Edwin Kemble,Raymond Birge, Robert Mullikan and John Slater developed a pragmaticAmerican style and instrumentalist attitude to chemical physics which washighly successful and closely related to empirical work in chemistry(Schweber 1990).

The German approach, on the other hand, was more of a mathematical-deductive nature. It was clearly oriented towards physical theory, and therewere almost no contacts to the chemists or attempts to engage in a fruitfulinterdisciplinary collaboration. It is characteristic that Born did not speak ofchemistry as a science on the same footing as physics, but described therelationship in a somewhat imperialist rhetoric. ‘‘We realise that we have notyet penetrated far into the vast territory of chemistry,’’ he wrote in 1920, ‘‘yetwe have travelled far enough to see before us in the distance the passes that

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must be traversed before physics can impose her laws upon her neighbourscience’’ (Born 1920, p. 382). The Göttingen physicists tended to think ofchemistry in a reductionist sense, as an immature field that could only beturned into a proper science by means of mathematical physics. Thus, in anunpublished praise of Debye – ‘‘the Newton of molecular physics’’ – Hilbertwrote that he, Debye, had got close to ‘‘what in far distance is believed to bethe foundation of a new mathematical chemistry [that has] been sought forso long.’’3 Sommerfeld and Bohr likewise wrote about chemistry as an areato be conquered by the quantum physicists. They indicated that it would onlybe a matter of years until chemistry was completely mathematised and thuswould become truly scientific in Kant’s sense.4 Understandably, many chem-ists found this arrogant and reductionist programme abhorrent. It only in-creased the barriers between traditional chemists and the new generation ofphysicists. The kind of molecular physics presented in the Born-Hückeltheory was certainly no invitation to the chemists. Understandably, they ig-nored the physicists’ abstruse calculations. This situation of the early 1920sdid not change much over the years. At the end of the decade, when quantummechanics had opened up new avenues in chemical physics, it merely reap-peared in new forms (Nye 1993, pp. 228–249).

Hückel wanted to resume contact with his former teacher, who was now inZurich. When Debye moved, he had wanted Hückel to follow him as his assis-tant, but in 1921 employment restrictions for foreigners prevented the reunion.A year later the situation had changed and Hückel went to Zurich to work asDebye’s assistant, a position he held until 1927. His assistantship with Born inGöttingen was taken over by Hund. Hückel’s stay with Debye in Zurich wasfruitful. It was here that he made his first important contribution to theoreticalchemistry in the form of the theory of strong electrolytes. Before proceedingwith this work, it will be appropriate, in order to better understand its back-ground and novelty, to digress with a brief historical expose of the developmentof ideas concerning the dissociation of strong electrolytes.

4. The anomaly of strong electrolytes, ca. 1900–1920

According to Svante Arrhenius’s ionic theory, one of the foundations of thenew physical chemistry of the 1880s, electrolytes were incompletely dis-sociated into ions and the degree of dissociation given by the law of mass

Before quantum chemistry 7

action in the form of Ostwald’s law of dilution. If a denotes the degree ofdissociation and c the total concentration, the law states that ca2/(1–a) hasa constant value. The quantity a can be written as L/L0, where L is theequivalent conductance and L0 the conductance at infinite dilution. It thenfollows that for highly diluted solutions (a∑1) the conductance decreaseslinearly with the concentration: L(c)ΩL0ªkc. However, already in the 1890sit was known from electrical conductivity measurements that strong electro-lytes do not follow Ostwald’s law. For example, according to measurementsmade by Friedrich Kohlrausch about 1900, the conductance of a diluted,strong electrolyte would vary with the concentration as L(c)ΩL0 – A!c,where A is a constant.

Remarkably, the anomaly persisted for more than a quarter of a century.5

Even more remarkably, the theory of solutions of Arrhenius, van’t Hoff andOstwald was easily able to co-exist with the anomaly, although it was wellestablished and seemingly in flat disagreement with the foundation of physi-cal chemistry. Most chemists chose either to ignore the anomaly or be satis-fied with purely empirical modifications of the dilution law that, with goodwill, could accommodate the discordant conductivity data for strong electro-lytes. These attempts to save the phenomena explained them away ratherthan explained them. They remained within the paradigm of the new physicalchemistry, which was – so it was thought – incompatible with ideas of com-plete ionic dissociation. Such an attitude may seem overly orthodox but canbe explained in part by the social interests of the new group of physicalchemists. They were engaged in a struggle to win recognition for the ionictheory, which was violently attacked by many chemists. Under such circum-stances they were not eager to publicise discordant results and hang dirtylinen for all to see (Servos 1990, p. 123).

Yet in the long run heretical thoughts about the strong electrolytes couldnot be avoided, and in the first decade of the twentieth century a few physi-cists and chemists did advocate complete dissociation, although hesitatinglyand rather unconvincingly.6 Niels Bjerrum was the first chemist who clearlyargued the case of complete dissociation in strong electrolytes. In 1909 hesuggested that the law of mass action did not hold for strong electrolytes andthat measurements of conductivity and freezing point depression must beattributed to effects of interionic forces. Seven years later he provided moredetailed arguments for the idea of complete dissociation, this time incorpor-ating the theory of interionic forces that the British physicist Samuel Milner

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had recently developed. However, neither Milner’s theory nor Bjerrum’s argu-ments were generally accepted and they were either ignored or rejected byprominent physical chemists such as Arrhenius and Nernst. More successfulwas a theory suggested in 1918 by the Indian chemist Chandra Ghosh, whoassumed strong electrolytes to be completely dissociated into ions that werearranged in a space lattice like that of a crystal. Ghosh’s theory was widelydiscussed and for a brief period considered the solution of the anomaly ofstrong electrolytes. However, in 1921 it was discredited by critics who pointedout a number of serious flaws in the theory. In particular, whereas there wasgood empirical evidence for a square root relationship between conductivityand concentration, Ghosh’s theory led to a cube root relationship.

The situation in the summer of 1921 was, then, that the theory of completeionic dissociation of strong electrolytes had won increasing acceptance andthat many chemists had come to accept the role played by interionic forces.But there still was no quantitative theory of strong electrolytes that builton these ideas and was able to reproduce experimental data satisfactorily.Moreover, in spite of an increased receptiveness of chemists to the notion ofcomplete dissociation, it was still a controversial idea. What was needed wasclearly a theory which was not only mathematically detailed but also gaveresults in convincing agreement with experiments. When Debye in the sum-mer of 1921 heard the Swiss physicist Edmond Bauer give a talk on Ghosh’stheory and the unsatisfactory situation in the understanding of strong elec-trolytes, he decided to take over where Ghosh and Milner had left and putan end to the confusion (Debye and Hückel 1923a, p. 185; Snelders 1986a,p. 88). In particular, he realised that the non-random distribution of ions ina solution was the key element, but also that Ghosh’s spatial structure of ionsdid not allow for the statistical character of the distribution of ions.

5. Strong electrolytes explained: the Debye-Hückel theory

The basic ideas of the Debye-Hückel theory seem to have originated withDebye, who had already made important progress with the new theory beforehe was joined by Hückel in the summer of 1922. Hückel, with characteristicmodesty, always later referred to ‘‘Debye’s theory’’ rather than the ‘‘Debye-Hückel theory.’’ At the end of 1921, Debye reported his first results to theZurich Physical Society, including a preliminary calculation that showed the

Before quantum chemistry 9

conductivity of strong electrolytes to vary as the square root of the concen-tration. However, he realised that a more complete theory would require elab-orate calculations and might best be done in collaboration with an assistant.So when Hückel came to Zurich, he was at once put to work on the theoryof strong electrolytes, an area of research which was new to him but whichhe quickly mastered. The calculations were indeed elaborate and probablytook longer than expected, for it was only in early 1923 they were completed.The first of Debye and Hückel’s two papers on the subject appeared in theMay 1 issue of Physikalische Zeitschrift, to be followed by a sequel threemonths later (Debye and Hückel 1923a, 1923b). I shall not outline the con-tent of the Debye-Hückel theory but merely indicate its general character(for details, see Falkenhagen 1932). According to Debye and Hückel, themicrostructure of the solution is not completely random, but neither is itcompletely regular as in the case of Ghosh’s lattice theory. In average therewill be more ions of the opposite charge than ions with the same charge in avolume element surrounding a given ion. The result is that the ion is sur-rounded by an oppositely charged ionic cloud or atmosphere. Debye andHückel showed that the effect on the central ion was the same as if it wassurrounded by a sphere of opposite charge homogeneously distributed on itssurface. As the concentration increases, the radius of the sphere decreasesand the ionic atmosphere exerts a greater influence on the central ion. Basedon this picture, Debye and Hückel were able to calculate how changes inconcentration, through changes in the ionic atmospheres, affect the osmoticpressure, conductivity and other properties.

The model of Debye and Hückel had much in common with that of Milner,a similarity acknowledged by the two Zurich physicists. Indeed, the theorywas occasionally referred to as the Debye-Milner theory, whereas the termDebye-Hückel theory was sometimes restricted to its application to concen-trated solutions (Falkenhagen 1932). The mathematical treatment of Debyeand Hückel was however quite different from Milner’s. It was more trans-parent and, at the same time, more effective. By making clever use of approxi-mations and averaging procedures, the two physicists were led to definite andtestable results. For example, Debye and Hückel derived a formula for thevariation of the electrical conductivity with the concentration, which forhighly dilute solutions gave the simple square root expression LΩL0ªA!c.As Debye and Hückel stated in their conclusion, ‘‘In this way we obtainKohlrausch’s law, found empirically many years ago, ... . Moreover, the pro-

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portionality factor receives a natural interpretation in molecular terms’’ (De-bye and Hückel 1923a, p. 206).

In general, the new theory agreed nicely and persuasively with measure-ments, not only of conductivity but also of osmotic effects, solubility, andfreezing-point depressions. For example, it predicted that a semilogarithmicplot of the solubility for a given salt against the square root of the solution’sionic strength should approach a straight line in dilute solutions. The slopeof the line would be given solely by the valence type of the salt. This predic-tion was strikingly confirmed by data obtained by Johannes Brønsted andVictor La Mer in 1924 (Brønsted and La Mer 1924).

6. The impact of the theory of Debye and Hückel

It is instructive to compare the Debye-Hückel theory with the slightly earlierBorn-Hückel theory of polyatomic molecules, although the subjects of thetwo theories were of course entirely different. Both theories were mathemat-ically elaborate, but there was a great difference in the use of mathematics,which reflected the difference in scientific style between Debye and Born.While Born tended to cultivate calculations for their own sake and place lessemphasis on their connection with experiment, Debye’s approach was differ-ent. Although trained in the Göttingen style of mathematical physics, hisemphasis was on physics – or chemistry – not mathematics. Debye had ahighly developed physical intuition which he used to suggest simple modelsfrom which he would try to understand physical and chemical phenomenaand compare the results of his calculations with experimental data. It is evi-dent that Hückel’s pragmatic approach was much closer to Debye’s than toBorn’s, and also that Hückel held Debye in great respect both as a scientistand a human being. The memorial article that Hückel wrote about Debye in1972 speaks of a relationship which was closer than between a professor andhis assistant (Hückel 1972b).

In spite of its complexity, the style of the Debye-Hückel paper made itappealing also to chemists and helped breaking down the walls betweenphysicists and theoretical chemists. Debye and Hückel were physicists, andthey chose to publish their work in a physics journal rather than in theZeitschrift für physikalische Chemie, which would have been the naturalchoice for chemists. However, the fact that the theory originated in a physics

Before quantum chemistry 11

context did not mean that it was mainly discussed by physicists or that it hadno impact on the chemists. On the contrary, it was eagerly taken up by thechemists, most of whom recognised that here was finally the solution to theriddle of the strong electrolytes that had been sought so long. The impact ofthe new theory was enhanced by a comprehensive review that Hückel wrotein Ergebnisse der Exakten Naturwissenschaften in 1924 and in which he gavea full discussion of electrolyte theory suited for both physicists and chemists(Hückel 1924). The high evaluation of the Debye-Hückel theory may be illus-trated by the words of Samuel Glasstone, who in his influential textbook of1940 characterised the theory as ‘‘the most significant advance in electro-chemistry since Arrhenius enunciated the theory of electrolytic dissociation’’(Glasstone 1940, p. 956).

Yet not all chemists were happy with the successful incorporation of theidea of complete dissociation into a larger theory. Hückel recalled that Nernstdisliked the theory because it built on what he considered the illegitimateconcept of complete dissociation. In a conversation of 1925, Nernst accusedHückel of having written ‘‘only nonsense’’ and suggested him to retract(Hückel 1972a, p. 3). However, Nernst’s opposition was not typical for thechemical community. At any rate, he seems to have had second thoughts andsoon realised that it was him, not Hückel, who was wrong. In the 1926 editionof his Theoretische Chemie, Nernst included a full and sympathetic descrip-tion of the Debye-Hückel theory. Moreover, he started experimental workrelated to the new theory (Nernst and Orthman 1928). Another of the foun-ders of physical theory, Svante Arrhenius, resisted until his death in 1927 thenew picture of strong electrolytes. It is ironical that Arrhenius, whose ionictheory was so revolutionary in the late nineteenth century, ended as a con-servative when the theory was completed by a later generation. But then itwas neither the first nor the last time that a pioneer in science came to resistthe development he had himself initiated.

The Debye-Hückel theory was a beautiful example of a classical theorybuilding on well-established physical laws and avoiding concepts of an adhoc nature. It relied on electrodynamics, thermodynamics and statisticalphysics, but not on quantum theory or models of atomic structure. It was animpressive theory, but of course it did not claim to be the final word aboutsolutions of strong electrolytes. Shortly after its appearance, the theory gaverise to a minor industry testing it experimentally and refining it theoretically.Among those who took up the new theory and improved and generalised it

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were La Mer in New York, Hans Falkenhagen in Cologne, Ralph Fowler inCambridge, and the three Copenhagen chemists Brønsted, Bjerrum and KaiLinderstrøm-Lang (Falkenhagen 1932). Hückel continued for a while to de-velop the theory and in 1925 he applied it to highly concentrated solutionsof electrolytes. Comparisons with data for hydrochloric acid showed goodagreement between theory and experiment (Hückel 1925).

The most important of the extensions was undoubtedly that of Lars On-sager, who was only 22 years old when he came from Trondheim, Norway,to visit Debye and Hückel in 1925. When he entered Debye’s office, Debyeasked him what he wanted and, as Hückel recalled the encounter, the youngand unknown Norwegian straightforwardly said ‘‘Your theory is wrong’’(Hückel 1972a, p. 3). Onsager had not really proven the theory wrong, buthe had located a significant flaw in it. Debye and Hückel had calculated theconstant A in the expression for the conductance (as given above), but theirvalue did not agree well with experimental results. Onsager realised the rea-son for the discrepancy, namely, that Debye and Hückel had not treated themotion of all ions equally. By allowing all ions to undergo Brownian motion,he found a much better value for A (Onsager 1926). His conductance equa-tion had the more general form L(c)ΩL0 – (A π BL0) !c. Onsager’s theory,often known as the Debye-Hückel-Onsager theory, was essentially an exten-sion or refinement of the Debye-Hückel theory, not a refutation of it. It wasOnsager’s first step in a career in theoretical chemistry that forty-two yearslater would make him a Nobel laureate.

7. Conclusion

As a whole, Erich Hückel’s early years as a scientist, working in physics andtheoretical chemistry, were not particularly impressive. He had establishedfor himself a reputation as a skilful theoretician who mastered complicatedcalculations, and had made an important contribution to physical chemistry.However, by 1925 his list of publications was slim and he had done almostall of his work as a junior partner with other physicists. It would not beevident that he was a creative and original scientist and not merely a calcu-lator. Moreover, from the physicists’ point of view his work may have seemedsomewhat on the fringe of what really mattered, which at the time was quan-tum theory. More or less deliberately, Hückel missed the opportunity to be a

Before quantum chemistry 13

player in the first and exciting phase of quantum mechanics in which so manyof his young physics colleagues from central Europe made their marks.

Quantum mechanics originated in Göttingen, Hückel’s Alma Mater. Lessthan a year later, Erwin Schrödinger developed his wave mechanics in Zurichwhile Hückel was there and with Debye taking an interest in Schrödinger’sideas (Debye 1964). However, the nearness of the quantum-mechanical revol-ution seems not to have inspired Hückel to take up the new theory. At thattime he was thirty years old, reputedly a high age for a theoretical physicist.As Friedrich von Weizsäcker once remarked about this golden age of physics,‘‘it was very difficult not to feel senile after having lived thirty years.’’7

Erich Hückel was a modest person who tended to doubt, and even under-estimate, his own intellectual capacity. While at Götingen he naturally com-pared himself with Heisenberg, Jordan, Pauli and the other young geniuses,and the comparison may have frightened him. Rather than entering the com-petitive world of quantum mechanics he chose to work in more traditionalborderline areas between physics and chemistry. These areas lacked the glam-our of quantum mechanics, but he felt more secure and knew he could dogood work here.

From the point of view of frontier physics of the late 1920s it might seemthat Hückel had ended on a side track and that his further career lookedunpromising. He had failed to jump on the quantum mechanics band-wagonand instead worked with unglamorous fields such as electrolytes and colloids.But Hückel was merely late in blossoming. He didn’t feel senile at all anddeclined to follow the alleged rule that physicists’ creativity peaks before theyreach thirty. After studies with Bohr in Copenhagen and with Heisenberg inLeipzig he mastered quantum mechanics and was ready for his masterpiecesin quantum chemistry. It was his works of 1930–31 on the double bond andthe structure of the benzene molecule that really put him back on the maintrack and secured his reputation as one of the leading chemical physicists ofthe century (Berson 1996).

As noted by Jerome Berson (1996), Hückel’s works in organic quantumchemistry share methodological features with his works on electrolytes.Hückel was trained in the Göttingen tradition with its emphasis on rigorouscalculations from first principles, a tradition that reinforced the separationbetween theoretical physics and chemistry. His early work with Born exempli-fies a kind of theoretical chemistry which has almost no connection at all toempirical data. Only after Hückel left Göttingen to work with Debye in Zur-

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ich did he develop his own, more pragmatic style. Inspired by Debye, helearned to use mathematics and physics to construct models from whichchemical phenomena could be understood; and he learned to value approxi-mations and appeals to empirical data. Quantum chemistry, and theoreticalchemistry in general, owed much to scientists who realised the importance of‘‘escaping from the thought forms of the physicists,’’ as the British quantumchemist Charles Coulson phrased it (Gavroglu and Simoes 1994, p. 109). Thisis an apt characterisation also of Erich Hückel’s early work in theoreticalchemistry.

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NOTES

1. The idea of liquid crystals dates back to 1889 when the German physicist Otto Lehmanexamined cholesteric esters and concluded that they, although liquid, had crystallineproperties. The claim was rejected by the Göttingen solid-state chemist Gustav Tam-mann and resulted in a 30-year-long controversy (Knoll 1988). The substance examinedby Hückel turned out not to be a crystal.

2. Archive for History of Quantum Physics (Niels Bohr Archive, Copenhagen). Interviewby Thomas Kuhn, June 26, 1963.

3. Draft for a letter of January 25, 1915 (Schirrmacher, forthcoming). Hilbert first wrote‘‘Newton of chemistry,’’ but crossed it over and replaced ‘‘chemistry’’ with ‘‘molecularphysics.’’ The German sentence is ‘‘... die solange vergeblich gesuchte in weitere Fernegeglaubte Grundlage einer neuen mathematische Chemie.’’ I am indebted to ArneSchirrmacher for providing me with the text of the German original.

4. See, e.g., Bohr 1922 and Sommerfeld 1922, p. 88. For Kant’s view of chemistry and therelationship between chemistry and physics in the period, see Nye 1993.

5. For historical details and references to the literature, see Wolfenden 1972 and Snelders1986a, 1986b. A contemporary review of the development of the theory of electrolytesshortly before the breakthrough of Debye and Hückel is given in Auerbach 1922.

6. References to about a dozen scientists who vaguely suggested the idea of completeionisation between 1896 and 1916 can be found in Wolfenden 1972, Servos 1990, pp.125–34, and Falkenhagen 1932, pp. 91–96.

7. Archive for History of Quantum Physics (Niels Bohr Archive, Copenhagen). Interviewby Thomas Kuhn, July 9, 1963.