by Thomas Hager
One insight he worked on while in Cambridge owed something to biology. Pauling had been attending biology seminars at Caltech, and had been interested to see how the geneticists mapped the location of genes on chromosomes by measuring how frequently two independent traits were inherited together: The closer the two genes were, the greater the chance they would stay linked together during genetic crossover. Pauling now borrowed the mapping idea to create his own scale of the relationship between pairs of elements. The more ionic character he calculated in the bond between two atoms, the greater the difference in their ability to attract electrons, and the farther apart they were on his scale. Fluorine, for example—the most electron-hungry of all elements—was at the far end of the scale. Lithium was toward the other. The bond in the compound they formed, lithium fluoride, was almost 100 percent ionic. Iodine was somewhere toward the middle of Pauling's scale, and the lithium iodide bond therefore had more covalent character. By comparing a number of such pairs, he was able to map a relative property he called electronegativity and assign values to various elements. These values in turn could be used to predict bond type and strength in many molecules, including those for which no experimental data were available. Between lectures at MIT, he wrote up his ideas in a paper that would become another in the "Nature of the Chemical Bond" series, finishing it just a few days before boarding the train back to Pasadena. He arrived home on May 30, one day before the birth of his new daughter, Linda Helen.
Pauling's electronegativity scale was one of his least theoretically well founded ideas and one of his most influential. It was a number of steps removed from any rigorous grounding in quantum mechanics but was easily grasped by chemists, who recognized its practicality in addressing real-world problems. By comparing the electronegativity of two elements from his tables, researchers could for the first time roughly predict the properties of a bond formed between them without having to know the first thing about the wave equation. The scale quickly earned wide adoption.
Pauling's mixing and matching of empirical observations with ideas from quantum physics was imaginative and dangerous. At each step he added a few more assumptions and moved a little further away from a hard grounding in accepted theory. Years later this would trip him up when critics would question the justification for his schemes, but for now it appeared that everything he did worked, and worked brilliantly.
Or almost everything. One daring prediction Pauling made was that fluorine was so electronegative it would form compounds even with an inert gas like xenon. Inert gases of any sort were thought incapable of chemical combination (that was, after all, why they were called inert) and the creation of a xenon compound would have made history. Experiments were needed to test his prediction. Pure xenon gas was extremely rare, but Pauling managed to obtain a little of it from a colleague and gave it to Yost, who worked through the summer of 1933 searching for the predicted compounds.
He failed to find any—a failure that Pauling found both confusing and galling. The reasons for Yost's inability to find what he was looking for are uncertain. However, thirty years later, another team would make international news by producing the xenon compounds Pauling had said were possible.
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The most public demonstration of the power of Pauling's resonance ideas came when he used them to solve one of the oldest problems in organic chemistry.
Benzene was a conundrum. It is composed of six carbon atoms and six hydrogen atoms, but the structure of the benzene molecule had eluded definitive analysis. In the winter of 1932-33, Pauling and his student George Wheland set out to solve the structure according to the concept of resonance. By spring they finished a paper, the fifth in Pauling's chemical-bond series, in which benzene was described as resonating between five extreme, or "canonical," structures. "The properties of the molecule," Pauling wrote, "would then be expected to be a sort of average of the properties of the individual molecules." Pauling and Wheland's approach seemed to work: The values calculated from their resonating structure fit what was known about the molecule's structure, reactivity, and stability. They expanded their approach to other aromatic molecules like naphthalene (using no fewer than forty-two canonical structures) and to hydrocarbon free radicals.
Pauling's benzene paper marked an important expansion into the realm of organic chemistry. A good deal of his subsequent work would focus on organic molecules, and George Wheland would go on to build his scientific life around it, in 1944 publishing an influential textbook, The Theory of Resonance and Its Application to Organic Chemistry, dedicated to "my first and greatest inspiration," Linus Pauling.
The powerful concept of resonance was now entering full flower. The last two entries in Pauling's nature of the chemical bond series, written in 1933 with postdoctoral fellow Jack Sherman, extended it to more chemical puzzles involving variations from classical single, double, and triple bonds. This work, too, was groundbreaking. Pauling showed that molecules were not restricted to whole-integer bonds but that the links between them could take on intermediate forms. Here again he melded the bond lengths and strengths from his ever-growing library of molecular structures with his ideas about the stabilizing influence of resonance, and again he came up with novel explanations for a whole slew of problems. Atoms connected by single bonds were known to be able to rotate like wheels on an axle, for instance, while those linked by double and triple bonds were held rigidly in place; Pauling postulated that those intermediate between single and double bonds—with what he called "partial double-bond character"—were also restricted from rotating, an important factor in predicting structures. Pauling explained the restriction on rotation in quantum-mechanical terms, then applied his idea of resonance between single and double bonds to explain the bond lengths and rotational properties of a number of intermediate cases.
All of chemistry began to re-form itself in Pauling's mind. He was creating a new kind of resonance, between quantum-mechanical theory and chemical reality. Like a jazz musician, he was taking themes suggested by quantum mechanics and improvising on them with his semi-empirical variations. His was a new kind of chemistry that played in the spaces between old categories. Pauling's quantum chemistry was not either/or: either this or that orbital, either ionic or covalent bonds, either single or double links. Pauling's chemical bond was a fluid, multiform thing. This was exciting, beautiful music, and he was the first to play it.
The Optimist
In the early 1930s, Pauling published an average of one significant piece of work every five weeks, most of it on the chemical bond or new molecular structures. By the end of that period he had moved almost entirely away from wrestling with the wave equation: "About 1933 or 1934 I gave up on the idea of myself making very complicated quantum-mechanical calculations about molecular structure," he said. "I made a lot of simple quantum-mechanical calculations and drew conclusions, and realized that if you could ever make really accurate quantum-mechanical calculations you wouldn't learn anything from them because they would just agree with the experiment."
Using his semi-empirical approach, he racked up success after success, until, by 1935, he wrote, "I felt that I had an essentially complete understanding of the nature of the chemical bond."
Slowly, his new vision of chemistry began to be accepted by other chemists. The reasons were manifold: the open and accepting atmosphere for new ideas at Caltech, his ability to speak the language of chemists, his eagerness to travel widely to spread his ideas, his unique ability to blend structural studies and quantum theory, and his courage in publishing theoretical insights without a rigorous grounding in hard mathematics. But one reason stood out above all: his optimism. In 1935, two observers describing the recent advances in quantum chemistry for the Review of Modern Physics could have had Pauling in mind when they wrote, "To be satisfied, one must adopt the mental attitude and procedure of an optimist rather than a pessimist. The latter demands a rigorous postulational theory, and calculations devoid of any questionable approximation or of empirica
l appeals to known facts. The optimist, on the other hand, is satisfied with approximate solutions of the wave equation. . . . He appeals freely to experiment to determine constants, the direct calculation of which would be too difficult. The pessimist, on the other hand, is eternally worried because the omitted terms in the approximation are usually rather large, so that any pretense of rigor should be lacking. The optimist replies that the approximate calculations do, nevertheless, give one an excellent 'steer' and a very good idea of 'how things go,' permitting the systematization and understanding of what could otherwise be a maze of experimental data codified by purely empirical valence rules."
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As his reputation grew, Pauling's life changed. In 1931, less than four years after taking his first appointment as assistant professor, Pauling was promoted to full professor at Caltech. By 1933 he oversaw twice the number of graduate students and postdoctoral fellows of any other chemistry professor in the division. He was awarded an honorary doctorate from his undergraduate institution, Oregon Agricultural College. As job offers kept coming in—Stanford, the University of London, Ohio State—his Caltech salary went up rapidly. His teaching load was reduced to one seminar per term, leaving plenty of time for research. Noyes was making it clear to everyone that the young dynamo was his "white-haired boy," as one colleague remembered it, and perhaps his successor as director of the Gates Laboratory. His annual teaching stints at Berkeley cemented his friendship with G. N. Lewis—they had "wonderful arguments," Pauling remembered, sitting in Lewis's smoky office—leading to talk for a while of writing a book on valence together. (The project never came to fruition.)
But to Pauling the most significant honor of those years was his election to the National Academy of Sciences. Noyes helped engineer it, and it was an important step up. The academy was the nation's most prestigious, exclusive, and hoary scientific club; in the early 1930s, out of all the thousands of scientists in America, there were only 250 members, most of them twice Pauling's age. When he was inducted in May 1933, Pauling became the youngest person ever elected in the seventy years since the academy's founding.
He was now, at age thirty-two, ensconced in the top echelon of the nation's scientists. He was young and famous and he was doing research he loved for a good deal of money. He had healthy children and was married to a devoted wife. He was being given almost everything he wanted at Caltech, was able to travel and talk almost anywhere he wished, and was producing first-rate work.
The warm sun of success and approbation evaporated any public traces of his adolescent shyness, and he began to earn a reputation as one of the most cheerful and extroverted of scientists. At departmental parties he became, like G. N. Lewis, the center of the liveliest group. He enjoyed a stiff drink, loved a good joke, even if it was a bit off-color, and he could be heard across the room, roaring with laughter, often at his own punch lines. Pauling's wit could have a sarcastic edge to it as well, and he became known for his ability to lacerate the work of others, especially stuffed shirts, slow thinkers, and researchers whose work he did not value.
He was having a very good time, and it showed. His lecturing style had moved from confidence to bravado. He would stride into a classroom, his long, wavy hair flying, eyes sparkling, and launch into a seemingly disconnected series of observations punctuated by lightning mental calculations, jokes about colleagues, and things he had read in the paper that morning. He would wave his arms in an imitation of a hydrogen atom, perform stunts with chemicals, draw cartoons of cannons firing photons at electrons. Sometimes he would lie down on the lecture table, "Roman style," his students called it, talking with his head supported on one hand. And somehow, through it all, he would weave a coherent, eye-opening lecture.
The physicist Martin Kamen remembered seeing Pauling in classic form during a visit to the University of Chicago in the mid-1930s, on a "wonderful Monday" when it was announced that the regularly scheduled noon-seminar speaker in physical chemistry would have to yield his place because Linus Pauling was in town:
Pauling arrived just before noon. We students were charmed, if slightly surprised, to see a bouncy young extrovert, wholly informal in dress and appearance. He bounded into the room, already crowded with students eager to see and hear the Great Man, spread himself over the seminar table next to the blackboard and, running his hand through an unruly shock of hair, gestured to the students to come closer. He noted that there were still some seats vacant at the table and cheerfully invited students pressing in at the door to come forward and occupy them. As these were seats reserved for faculty, the students hung back, but Pauling would have none of that. He insisted, and they nervously edged in, taking the seats. The talk started with Pauling leaping off the table and rapidly writing a list of five topics on which he could speak singly or all together. He described each in a few pithy sentences, including racy impressions of the workers involved. . . . The seminar he gave was a brilliant tour-de-force and made a never-to-be forgotten impression on all us students.
Pauling cared deeply about teaching, partly in reaction to what he liked and disliked about his own education, partly in response to his new understanding of his own science. He thought a chemical education should start with a sense of wonder. As early as 1930 he was advising Noyes to change the way freshman chemistry was taught at Caltech, to hold back on theories and mathematics until the students had a firm grasp on descriptive chemistry. "To awaken an interest in chemistry in students we mustn't make the courses consist entirely of explanations, forgetting to mention what there is to be explained," he said. "I know of no chemist who was attracted to this field because of theoretical chemistry. Instead, it is an interest in chemicals and their reactions which first attracted the chemist." He also proposed giving students drawings of molecules "as we now picture them," to give them a concrete feel for what they were studying. Such molecular drawings, now common in most chemistry textbooks, had not been used before.
More important, he thought chemistry should be taught not as a loose aggregate of facts but as a unified science with a firm and consistent underlying theory. His own ideas about the chemical bond could be used to explain a wide variety of phenomena, from thermodynamics to crystal structures, from organic to inorganic chemistry, providing a new level of order and sense, and he began organizing his classes around these basic themes.
The best of Pauling's students came away enthusiasts. However, those undergraduates, especially nonchemistry majors who lacked the necessary background, often found Pauling obscure.
He could be rough on students who he thought did not accord him proper respect. On the first day of one freshman chemistry course, Pauling's "Roman style" position earned him a loud laugh from the students. He was not amused. Pauling "lit on the biggest guy in the class—later tackle on the football team—and sent him out," remembered one 1933 student. "From then on, it was a very sober group of freshmen." Perhaps in some ways he still looked a little too young to be taken seriously. In the summer of 1934, Pauling grew a thick auburn beard, designed in part to give him a more mature, professional air, and maintained it, off and on, for a few years.
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As Pauling's career rocketed, his relationship with Ava Helen changed. She had once been able to help him with his work, making notes and drawing diagrams, helping to make crystal models. But as his work grew increasingly theoretical and sophisticated, she was left behind. "I helped in making the indexes of the books, and even in the beginning doing some proofreading," she remembered, "but he found people who could do this much better than I and without the effort that it took me. Moreover, I was developing my own interests more too, and my time was taken up in other ways."
The "other ways" mostly concerned children and home. By the end of 1932 the Paulings had moved to a larger house close to Caltech, which required more tending, and there were now three children to care for: the new baby Linda, one-year-old Peter, and the rambunctious seven-year-old Linus junior. As with most things, Ava Helen took to her ne
w responsibilities wholeheartedly. "If a woman thinks honestly and clearly," she wrote to herself in 1927, "she must soon reach the conclusion that no matter what life work she chooses it could be done better by a man; and the only work in which this is not the case is the work involved in a home with children." These ideas would eventually change. But for now, she fell back on her college training in a scientifically based form of home economics, and determined early in her married life to create an ideal home and raise ideal children.
The pattern of the Paulings' home life became set. Ava Helen spent her days in childcare and housekeeping, cooking and cleaning; Pauling spent his doing science. He had a small study at home where he worked in the early mornings. After breakfast he walked two blocks to Caltech and worked all day at the office and lab. He walked home for dinner, then either returned to Caltech for evening seminars or retired to his home study to do more calculations late into the night. He worked weekends. He worked most holidays. When he traveled, as he did often for meetings and visiting lectureships, he did so almost always alone, mostly by train, which he enjoyed because it offered him uninterrupted time for more work.
