The Public Response to The Nature of the Chemical Bond

Pauling lecturing on valence and molecular structure, 1957.

[Celebrating the 75th anniversary of The Nature of the Chemical Bond. Part 6 of 6.]

Not all of the responses to The Nature of the Chemical Bond that Linus Pauling received were from academics; some came from students.  Lois Joyce was one such respondent.  Working her way through graduate school at the University of Illinois, she began contacting Pauling in May 1939 in hopes that she could study with him at Caltech.  She told Pauling how she was much more interested in his focus on molecular structure than on the analytical chemistry that she was studying at Illinois.  By July, after bringing her case before the Division of Chemistry and Chemical Engineering at Caltech, Pauling was compelled to tell Joyce that he could not bring her aboard due to Caltech’s restriction on women without Ph.Ds working in their labs.  Pauling encouraged Joyce to continue on and get her doctorate at the University of Chicago, after which point she might be able to join him.

A month later, Pauling received a letter from Joyce’s mother asking for an autographed copy of what she could only remember as “The Strength of the Chemical Bond” for her daughter’s birthday.  She told Pauling how Joyce thought Pauling to be “one of the greatest men in the world” and that even though “it’s a strange career for a girl” her daughter was “deeply interested in X-ray research and willing to give up all pleasure in life to succeed,” often studying “until two and three in the morning.” Pauling obliged by signing and sending a copy of his book through special delivery, as Joyce’s birthday was only days away.  Joyce was overjoyed when she received the gift, telling Pauling that she was glad her mother “bothered” him to send a copy and that she was “never so thrilled.” Joyce still regretted that she was unable to work with Pauling however, telling him again that the only topic she wanted to study was molecular structure.


Scholarly reviews of The Nature of the Chemical Bond did not start appearing until 1940, the year following its publication.  Many of the reviews offered soaring praise not only for the book, but for Pauling as well.  They also included more technical criticism than had been contained in the letters that Pauling had received earlier from his colleagues.

The Transactions of the Faraday Society published a review, identifying the author as L. E. S., which played off the assumption that Pauling was already so well-known as to need little in the way of an introduction.  L. E. S. noted, “All who have researched in the field of molecular structure have long awaited this book.” According to the review, Pauling’s account mostly focused on “the structure of individual molecules…with problems centering around relatively normal covalent bonds.”  It also included a “delightful chapter” on the hydrogen bond and two “succinctly but excellently discussed” chapters on the structure of crystals.

Similar praise came from John E. Vance in the American Journal of Science and George B. Kistiakowsky in the Journal of the American Chemical Society, who both lauded Pauling’s non-mathematical style.  Kistiakowsky added that Pauling’s presentation was “by and large…lucid, and a student with little more preparation than the four basic courses in chemistry should be able to digest most of the contents.” Indeed, even novices might be interested and find the text “stimulating.” The reviews from Germany echoed these sentiments, though one author in the Zeitschrift des Vereins Deutscher Chemiker was disappointed that Pauling, like most English chemists, ignored the German literature on the subject.

As the reviewers continued, they brought in a handful of criticisms of Pauling’s work.  L. E. S. found that Pauling had oversimplified his discussion and gave it a “premature happy ending.” According to L. E. S., these flaws emerged from Pauling having written such a broad survey and resulted in assumptions along the lines of “the electric dipole moment of a purely covalent link is small or is zero,” a suggestion presented without any proof.  L. E. S. wrote that Pauling’s certainty in his own understanding came “partly from this simplification and partly from tricks of style.”

Kistiakowsky made a similar observation, calling attention to Pauling’s “pontifical style” and “his advocacy of the doctrine of infallibility of Pasadenean research,” before concluding that this was “understandable and should not be taken amiss.”

A more serious criticism came from Vance, writing in December 1939,  who complained that Pauling ignored a large stream of the chemical discourse then current, and in particular found it “remarkable that no mention of the Hund-Mulliken treatment of chemical bonds,” based on molecular orbitals, appeared in the text.  Such an inclusion, Vance thought, surely would have increased the “usefulness of the book.” He was not the only person who felt this way.

Robert Mullikan, 1929.

In June 1940, Robert S. Mulliken – himself a rising scientific star and future Nobel Prize winner whose atomic structure work was sometimes at odds with Pauling’s theories – published his own review in the Journal of Physical Chemistry.  Mulliken initially took a generous tone, calling Pauling’s book a “clearly written survey of the nature of the chemical bond,” before adding the critical clause, “from the viewpoint of the atomic orbital method.”  This viewpoint, Mulliken ceded, was most suited to combining wave mechanics with the “traditional ideas” of chemical bonds and therefore had “wide appeal and usefulness among chemists,” but it was not the only perspective in play: Mulliken’s molecular orbital model, developed with Friedrich Hund, was a competing body of work that Pauling largely ignored.

Mulliken suggested that Pauling’s failure to include a molecular orbital perspective, except for a brief “aside,” was misleading, especially for the “unfamiliar reader.”  “Most authorities,” according to Mulliken, “would feel that for a deeper understanding of the electronic structures of molecules a knowledge of both methods is necessary, and that for many problems the MO [molecular orbital] method is the simpler and more intelligible.” Despite this criticism, which became more common over the years, Mulliken ended on an upbeat note, echoing his earlier comment on the volume’s usefulness and recognizing that “the book is a landmark in the history of valence theory.”

While criticism, like Mulliken’s, urging Pauling to add another theoretical perspective to The Nature of the Chemical Bond may not have been exactly what Pauling was looking for from the public response to his book, he would not be able to incorporate any of this new feedback into the next edition anyway.  After he had received his requested interleaved copy from W. S. Schaefer of Cornell University Press back in October 1939, Pauling quickly got to work on making revisions.  By December he was already sending in the first three chapters to Schaefer and by May 1940 the second edition was out, published one year after the first edition and one month prior to Mulliken’s review, also of the first edition.  The opportunity to incorporate new suggestions was not completely lost, however, as Pauling began working on yet another version of his landmark text just one year later, in 1941.

Pauling's interleaved copy, full of notes for future revisions.

Pauling’s interleaved copy, full of notes for future revisions.

New Insights into Metals and More

Linus and Peter Pauling at Warwick Castle, England. 1948.

[The Paulings in England: Part 3 of 5]

In his lab, a five minute walk from his office at Balliol College (where he was once caught boiling an egg on his electric space heater), Linus Pauling’s research took a turn from the contents of his lectures – intermolecular forces and biological specificity – and he found himself devoting his research time to metal theory. Pauling had planned to revise the index for his newly published freshman text, General Chemistry, during his Eastman Professorship, but couldn’t seem to get metals off his mind.  As he wrote in a letter to his Caltech colleague J. Holmes Sturdivant, “I thought that I would be doing work in connection with my freshman text while in England, but it has turned out that I have devoted all of my time, and presumably shall continue to do so, to work on the theory of metals and intermetallic compounds.”

He was aided in his lab by three other researchers – David Shoemaker, Hans Kuhn, and a young man from Holland, Dr. F. C. Romeyn. Pauling’s circumstances were proving to be highly productive, and in a March letter to Robert Corey, Pauling wrote of the impact that the change of setting was having in stimulating his thoughts:

I have been having wonderful success in my development of a theory of metals. I think that it has really been very much worthwhile for me to get away for this period of time, under circumstances favorable to my thinking over questions and trying to find their solution. The problem of metals has been on my mind for a number of years, and I haven’t been able to leave it alone, so it is a good thing that I have now managed to get it solved.

This new theory of metals was an extension of Pauling’s valence-bond approach to determining the structure of molecules, as initially developed in the late 1920s. Pauling was first exposed to quantum mechanics as an undergraduate at Oregon State University (then known as Oregon Agricultural College) and retained that interest as he transitioned to graduate studies and faculty employment at the California Institute of Technology.

In 1926 Pauling traveled on a Guggenheim Fellowship to study the developing field of quantum mechanics with physicists in Europe, and especially Germany. He brought these new ideas back to Caltech in the form of quantum chemistry, which he used to compute the electronic structures of molecules. This intuitive valence-bond approach was quickly judged a success and had been popular since the 1930s as a simple model for studying the electron dispersal in the bonds between molecules.

But all the while another chemist, Robert Mulliken (recipient of the 1966 Nobel Prize for Chemistry) had been steadily fostering a rival approach: the molecular orbital theory. While the Pauling family enjoyed springtime in Paris at the beginning of April, Pauling and Mulliken met head to head at a conference on Isotopic Exchange and Molecular Structure. There an entire day was devoted to the comparison of the two theories before a group of quantum chemists. Pauling had written earlier that molecular orbitals were confusing to students, but he learned at this meeting that one always has to stay one’s toes: with more mathematics under their belts, advanced chemistry students were increasingly hungry for the more quantitative approach that Mulliken’s theory offered.

Sometimes ideas come upon the great thinker at surprising times, and Pauling experienced just such a eureka moment during one of his twice-weekly Oxford lectures in February.  As he wrote to Holmes Sturdivant,

I have just had a great stroke of luck. While giving my lecture on Tuesday I suddenly realized that a calculation about resonance energy of metals that I had just made and was reporting contained the key to the strange valence numbers and numbers of atomic orbitals and unused orbitals that have turned up in my theory of valency of metals.

Notes on intermetallic compounds by Linus Pauling, March 1948.

Pauling worked out his ideas on electron theory and the structure of metals and intermetallic compounds through pages and pages of careful handwritten calculations. In looking at each manuscript now, Pauling presents a hypothesis about some aspect of metal theory and then proceeds to calculate, revise, and recalculate until the theory and the experimental x-ray diffraction data line up. For instance, on one day in March, Pauling was exploring intermetallic compounds from several different angles.  He writes “I shall now treat intermetallic compounds, with my new ideas – resonance of bonds when an extra orbital is available, importance of n=1/2, 1/4 etc., concentration of bonding electrons into strong bonds (Zn-Zn, etc as compared with Na-Na) , transfer of electrons with increase in valence.” Hybrid orbitals, bond lengths, and the overall stability of structures were other items on Pauling’s research agenda.

Of course, not every idea is a winner and a few theories led Pauling down the wrong path; in one manuscript Pauling set out to, as he wrote, “consider sp hybridization – how can we set up a secular equation to give the results given by my bond-strength postulate?”  In the end Pauling found that “the ratio does not come out as desired. It is evident that my assumption that the energies can be taken proportional to ‘bond strengths’ is not right.”  Missteps such as these didn’t deter Pauling from pressing on with his research, for as he often said, “The way to have good ideas is to have lots of ideas, and throw away the bad ones.”

Chemistry boasts its own special language, or nomenclature, and chemists like Pauling are to thank for the terms that make chemical jargon unique. As research advances, sometimes an entire new word is needed to describe an innovative concept. While tackling the nuances of metal theory at Oxford, Pauling wrote to Sturdivant about this very problem.

By the way, I think that we should do something toward improving the nomenclature. For example, coordination number is an awkward and unwieldy expression – we need one short, precise word for this concept. Perhaps ligancy could be used. It would fit in well with ligand and the verb to ligate. We also need some general words to express the bonds between one atom and the surrounding atoms – we now use the word bond to refer both to the electron pair bond that is resonating around among alternative positions and to the fraction of an electron pair bond that is a portion to a particular position. I have also felt troubled about using the word position in this way – to mean the region between two atoms. If we do introduce any change in nomenclature, it must be very well thought out, and must not involve too great a strain on the memory, or too great a departure from the past.

New fields also call for innovations in instrument development and research programs. Pauling was in constant communication with his colleagues back home about new tools that might be constructed to aid the researchers. He admired the Cavendish’s vast x-ray crystallography laboratory and also gained new insights from reading British journals devoted to scientific instrumentation. He would frequently send word back as to how Caltech workers could improve on a complex apparatus such as the specialized cameras for x-ray diffraction of metallic crystals.

Pauling was likewise intrigued by the English system of graduate education, wherein graduate students would take class work completely during the first year and then spend practically 100% of their time on research during the other two years. Pauling was always looking to improve upon existing programs, but as appealing as the English system was, he acknowledged that in implementing it one would run the risk of not knowing whether a student was an apt researcher for their entire first year!