Pauling’s Last Year as a Grad Student

Ava Helen and Linus Pauling, 1924.

Ava Helen and Linus Pauling, 1924.

[Part 3 of 3]

Pauling’s final year of graduate school at the California Institute of Technology, 1924-1925, was quite busy.  During this last phase of his student experience, Pauling’s primary research interests centered on hematite, corundum, and beta-alumina, though a great deal more professional and personal growth can be traced to this time in the budding young scholar’s life.

In his work on corundum and hematite, Pauling was assisted by Sterling B. Hendricks, a Texan who had received his master’s degree from Kansas State in 1924 was now in Pasadena, working on his PhD.  Hendricks became a close associate and personal friend of Pauling’s and, with their mentor Roscoe Dickinson away on a research trip, Pauling became Hendricks’ unofficial adviser. Such was Pauling’s influence that, later in life, Hendricks would come to consider himself to be “Linus’s first student.”

Together, Pauling and Hendricks worked on a theoretical paper that pieced together much of the work that they had completed over the previous year and a half. The paper was published in the Journal of the American Chemical Society (JACS) in March 1926 (nearly a year after Pauling had completed his PhD) and titled “The Prediction of the Relative Stabilities of Isosteric Isomeric Ions and Molecules.”  The paper was a milestone in that it was Pauling’s first paper devoted solely to the subject of the chemical bond.

It was not, however, the first paper that Hendricks and Pauling had co-authored. In 1925 the duo worked together to publish two sets of crystal structures: “The crystal structures of hematite and corundum” (March 1925) and “The crystal structures of sodium and potassium trinitrides and potassium cyanate, and the nature of the trinitride group” (December 1925).  During his last year of grad school, Pauling also collaborated with his friend and former roommate, Paul Emmett, on an X-ray determination of the crystal structure of barite.  Their article, which was published in JACS in April 1925, is another example of Pauling’s work that corrected previous published structures.

Peter Debye, 1926.

Peter Debye, 1926.

On top of the research that he was doing on crystal structures, Pauling also toyed with an idea in which he applied the Debye-Hückel theory, which was used to determine the energy coefficient of ions in dilute solutions. When he learned of this work, A.A. Noyes invited Peter Debye, who was based in Switzerland, to visit Caltech, in part to have him discuss his theory with Pauling. And although Pauling never published his original idea, in July 1925 Debye and Pauling did co-author a different paper, “The Inter-Ionic Attraction Theory of Ionized Solutes.  IV.  The Influence of Variation of Dielectric Constant on the Limiting Law for Small Concentrations.”  Appearing in JACS, the article was a contribution to a larger series published by the journal on the inter-ionic attraction theory of ionized solutes.


Later on in his life, Pauling developed a reputation for staying on top of the latest findings and issuing an informed opinion on a wide range of scientific topics.  This character trait was likely spurred by an experience that he had as a graduate student.

Early on in his graduate career, one of Pauling’s more influential professors, Richard C. Tolman, posed to him a question about diamagnetism. Pauling responded that diamagnetism was just a general property of matter, a lackluster reply that made clear that Pauling had not stayed current with the literature. Tolman kept questioning Pauling for more specific details until Pauling finally answered, “I don’t know.”  For this he was reprimanded by a Caltech post-doc who told him, “You are a graduate student now, and you’re supposed to know everything.” This was advice that Pauling took to heart and that made a big difference throughout his career in science.


The Paulings, 1925.

The Paulings, 1925.

Nearing the end of his graduate school tenure, Pauling read G.L. Clark’s paper on uranyl nitrate hexahydrate and, as he went, he corrected it.  This was a continuation of the critical reading habits that he had first developed at Oregon Agricultural College and had continued to hone by lantern light while working for the Oregon Highway Department the summer prior to his enrollment at Caltech. It was likewise a practice that he would continue throughout his career: closely reading papers and correcting errors, often by letting the author or publisher know what he had found.

By this time, with Roscoe Dickinson away, Pauling had taken up some of his mentor’s responsibilities in the lab and, as with Sterling Hendricks, was serving as an ad hoc advisor to several students.

Likewise, with Dickinson gone, Pauling began to develop his own techniques to aid in crystal structure determinations. A methodology that was quite different from the formal instruction that he had received, Pauling’s approach used atomic sizes and chemical behaviors to approximate reasonable structures for molecules.  After determining these possible structures, Pauling then used X-ray data to eliminate unlikely possibilities and to isolate the best possible structure for a particular substance.  As it turned out, this approach to scientific inquiry already had a name, the stochastic method, and Pauling ultimately put it to effective use across many different disciplines.


Linus Jr. and Ava Helen, 1925.

Linus Jr. and Ava Helen, 1925.

Pauling’s last year as a grad student also included big changes in his personal life.  After marrying in the summer of 1923, Ava Helen Pauling moved to Pasadena with her husband and kept house while he finished his degree. In the early years of their marriage, these duties also routinely included helping “keep house” in the laboratory, particularly by recording data and taking notes. Pauling’s research notebooks from these years are full of her handwriting, even including one note reminding Linus that she loved him.

In the midst of all his coursework and research, and as Pauling was wrapping up his last Winter term at Caltech, another big change came about when the Paulings’ first child, Linus Jr., was born on March 10, 1925.  By this time, Ava Helen was mostly excused from laboratory duty and focused her energies primarily on raising her children (ultimately there would be four) thus creating an atmosphere at home in which Linus could be as productive as possible.


Graduation day, 1925.

Graduation day, 1925.

Linus Pauling completed his PhD in chemistry in June 1925, tacking on minors in physics and mathematics as well. His dissertation, titled “The Determination with X-rays of the Structure of Crystals,” consisted of a compilation of articles that he had previously published with little more than new pagination connecting them together as a whole.

The summer after graduation, A.A. Noyes helped Pauling to secure a research fellowship that would enable him to stay at CIT and complete a research study on complex fluorides.  Pauling continued in this vein for the next eight months, during which time he began to make plans to leave Caltech to study as a post-doc at Berkeley, where he thought he might pursue a new set of experiments in G.N. Lewis’ lab, using funding from a National Research Fellowship that he had received.

Not wanting to lose Pauling to Berkeley and Lewis, Noyes managed to arrange for Pauling to remain in Pasadena in order to complete additional unfinished work on crystal structures.  Fortunately for Noyes, at the end of 1925, when the Guggenheim Fellowships were announced, Pauling was finally chosen for funding, having at last reached the program’s required minimum age.  At Noyes’s urging, Pauling resigned from his National Research Fellowship once he had received the good news from the Guggenheim Foundation. From there, Linus and Ava Helen took an important trip to Europe and ultimately returned to Caltech, their institutional home for the next thirty-six years.

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On Isosteric Isomers: An Important Early Paper

Ava Helen, Linus Jr. and Linus Pauling on a family hike, 1926.

In 1926 life was going well for twenty-five year old Linus Pauling – he had been married for a couple of years, had a healthy one year old son, and was quickly establishing himself as one of the top chemists in the United States. His primary research topic at the time was structural chemistry, and his hard work in the laboratory had already resulted in a good number of publications.

Nonetheless, Pauling had yet to publish a paper dedicated entirely to the subject that would soon become synonymous with his name: the chemical bond. This finally changed in 1926, when he and Sterling B. Hendricks  (who considered himself to be “Linus’ first student”) published a paper titled “The Prediction of the Relative Stabilities of Isosteric Isomeric Ions and Molecules.”

In this publication, Pauling and Hendricks calculated the potential energies of similar ions and similar molecules in order to predict the most favorable structure of a given ion or molecule. For example, the atoms in carbon dioxide (CO2) can only be arranged in two configurations, OCO and COO. By comparing the potential energy values measured for each structure, it is possible to determine which is more likely to naturally form – the structure with the lower potential energy.

In their 1926 paper, Pauling and Hendricks first found it necessary to define the terms “isosteric” and “isomeric.” Isosteric refers to “molecules or ions that contain the same numbers of atomic nuclei and the same numbers of electrons, but differ in the positive charges on the nuclei.” Some examples of isosteric molecules are:

:N:::N: :C:::O: (:C:::N:) (:C:::C:)

Isomers, on the other hand, are molecules or ions that contain the same atoms in a different arrangement. In the carbon dioxide example given above, COO is one isomer and OCO is another. Other examples include the cyanate ion (NCO) and the fulminate ion (CNO) as well as the NON and NNO configurations of nitrous oxide (NO2).

Although two molecules or ions that are isosteric aren’t always isomeric and vice versa, Pauling and Hendricks were only interested in substances that fulfill both conditions. Because isomeric and isosteric substances are so similar in many ways, Pauling and Hendricks argued that their differences in stability could be attributed almost exclusively to differences in potential energy. Using a rather complex equation that is explained in the paper, Pauling and Hendricks determined the most stable structures for a variety of isomeric and isoteric substances. Their results are displayed in the table reproduced below.

As the authors stressed in their paper, two points are particularly important to note: 1) the significance of relative values (as opposed to absolute values) of potential energy, and 2) that the configuration with the lowest potential energy is favored. Therefore, for carbon dioxide, the OCO configuration is favored over COO; for nitrous oxide, the NNO configuration is favored over NON, and so on. As it turned out, the two men’s results agreed very well with prior experimental and chemical evidence, with a few specific exceptions.

Although this paper is not generally counted among Pauling’s most important contributions, it does stand as an undoubtedly strong start to the chemical bond work that would win him the 1954 Nobel Prize in Chemistry. For more information on Pauling’s breakthroughs in structural chemistry, please visit the website Linus Pauling and the Nature of Chemical Bond: A Documentary History.

The Crystal Structures of Corundum and Hematite

Corundum model, side view.

Corundum, Al2O3 (aluminum = silver; oxygen = red). A hexagonal (rhombohedral) crystal system constructed of aluminum atoms that are each surrounded by six oxygen atoms. The oxygen atoms are not bonded at the corners of a regular octahedron.

Hematite model, side view.

Hematite, Fe2O3 (iron = orange; oxygen = red). A hexagonal (rhombohedral) crystal system constructed of iron atoms surrounded by six oxygen atoms not at the corners of a regular octahedron.


In 1925 Linus Pauling and Sterling Hendricks published a paper detailing the crystal structures of corundum and hematite. It was the fifth crystal structure analysis that Pauling had undertaken. During the early years of his research, Pauling had a tendency to correct the work of others, and the determination of hematite and corundum’s crystal structures was not an exception.

In 1917 the British father and son duo of William Henry and William Lawrence Bragg had studied the structure of ruby using X-rays. Citing this data, they hypothesized in 1924 that each aluminum atom in ruby is equidistant from six oxygen atoms, and that each oxygen atom is equidistant from four aluminum atoms. The Bragg’s used this hypothesis in their later work on theories of birefringence (the refraction of a ray of light into two slightly different and unequal rays) and to explain the intensity of X-ray reflections, in terms of temperature variation, from the faces of ruby crystals.

Hendricks and Pauling were not certain of the Bragg’s methods, and wrote in their analysis of corundum, “an exact knowledge of the arrangement of the constituent atoms in ruby would make the arguments of these papers much more convincing.” (J. Am. Chem. Soc., 47 (1925), p. 781)

Corundum is a gemstone whose varieties include ruby and sapphire. It is an aluminum oxide, and the second hardest mineral known to science after diamond. This property is generally attributed to the strong and short bonds which pull oxygen and aluminum atoms close together, making the crystal unusually hard and very dense.

Corundum model, top view.

Hematite comes in many varieties, each having their own unique name and composition. Hematite is an iron oxide, and very important as an ore of iron. It is also used as a pigment and is collected as a mineral specimen. It is blood red in powdered form, but can be gray, black, red or brown in its solid form. It is also used in jewelry, either as a set stone, or as a piece itself.

Pauling and Hendricks used Laue and spectral photographs, as well as the theory of space groups, to analyze the crystal structures of hematite and corundum. They found that, contrary to the Braggs’ hypothesis, the spacing of atoms in corundum’s atomic structure was not equidistant. Though they confirmed the Braggs’ ratio of oxygen to aluminum atoms, they found that instead of forming the corners of a regular octahedron around aluminum atoms, three of the six oxygen atoms were closer to the aluminum atom than were the other three. Similarly they found that instead of forming the corners of a regular tetrahedron around oxygen atoms, two of the aluminum atoms surrounding each oxygen atom were closer than the other two.

Sir William Lawrence Bragg

Pauling and Hendricks disproved the Braggs’ hypothesis of a constant aluminum-oxygen distance, and found that the Braggs’ value for the distance between aluminum and oxygen was also incorrect. The publication of the Pauling-Hendricks findings and the professional implications of their critique were not missed by the Braggs. Pauling was later told that Lawrence Bragg resented his “intrusion” into the fields of crystallography and mineralogy, and that he considered Pauling to be a competitor. Consequently, many of Pauling’s initial publications, often critiques of the work that others had done, led to the start of what would become a long lasting rivalry between himself and the Braggs.

Pauling later claimed that his view of their relationship was very different, both at the beginning of his academic career and the end of it.   According to Pauling, the work that was initiated to correct the Braggs’ early hypotheses was done in order to strengthen the validity of their subsequent claims. In regards to the influence of atomic arrangements on birefringence, this work was successful.

Pauling had perceived the early relationship between himself and W. L. Bragg as that of professor and student, respecting the work that the Braggs had done, and acknowledging that it had enabled him to study crystallography and chemical bonds upon his entrance to Caltech. In reference to the rivalry perceived by the Braggs, Pauling wrote “I did not think of my own scientific work as being competitive; I found it engrossingly interesting for its own sake.” Overall it seems that Pauling and the Braggs were not merely separated by an ocean, but also by an unfortunate misunderstanding of motives.

Pauling research notebook entries on the structures of corundum and hematite.

More on Pauling and Hendricks’ determination of the structures of corundum and hematite can be found in Pauling’s Research Notebook 4.   The larger story of Pauling’s structural chemistry work, including his relationship with the Braggs, is told in Linus Pauling and the Nature of the Chemical Bond: A Documentary History.