Who Wrote 78 Steps Health Us

proud to be an academic colleague of the men who had taught him just two years earlier.

And Pauling quickly proved his abilities. In the classroom, he became known as an exciting and stimulating teacher, as well as a leading expositor of the newest ideas in quantum physics. But his real proving ground would be research. Pauling's former professor, Roscoe Dickinson, put him in charge of running the X-ray crystallography program, where he set Sturdivant to work solving new crystal structures. Pauling finished cowriting a book about spectroscopy with Samuel Goudsmit, a young Dutch doctoral student he had met in Europe. He wrote a long scientific article that introduced American chemists to the

Nobel Prize-winning physicist Robert Millikan served as president of Caltech for more than 20 years. Under his leadership the school grew into one of the leading research institutes in the country.

Heitler-London approach to the chemical bond—and added some new ideas of his own.

Pauling worked hard to extend Heitler and London's ideas to more complex molecules. But he was immediately faced with a mathematical roadblock. Heitler and London's approach depended on applying Schrôdinger's difficult wave equation to every electron in a molecule—a daunting process even for the interaction of two simple atoms that quickly became impossible for the interaction of more complex atoms.

While he was working on this problem, another one caught his attention. X-ray crystallography was impossible to use on any but the simplest chemical structures, because anything more involved produced complicated X-ray diffraction patterns far too intricate to decode in the days before computers.

In both cases, Pauling needed to find approximations and shortcuts to help with the difficult mathematics. And here he discovered a talent that would be a key to his success as a scientist: He was very good at simplifying things.

Pauling's unusually powerful memory enabled him to store vast amounts of information, to build huge mental libraries of facts from chemistry, X-ray crystallography, mathematics, and physics. Rather than getting lost in the piles of facts and figures he had memorized, Pauling had the ability to focus on a particular problem and pull out the information relevant to it—rather like today's computer database programs, which, given the right keywords to search for, can sift through enormous amounts of information and pull out needed records.

Access to information was not enough by itself, however. The gathered facts also had to be put together in the right way to form an answer. In 1928, Pauling applied himself to solving the complicated structures of an important family of minerals called the silicates, which include topaz, talc, and mica. He knew it was likely that these minerals were composed of basic repeating units that included dozens of atoms, far too large a problem to solve directly with X-ray crystallography. So he began working at it the other way, from the bottom up, by thinking not about the whole daunting structure at once but breaking it down into basic pieces.

Pauling knew which atoms were involved. Silicates were made mostly of silicon and oxygen with a sprinkling of metal atoms. In England, one of the pioneers of X-ray crystallography, William Lawrence Bragg, had also been studying the silicates. Bragg thought of the atoms in silicates as though they were different-sized marbles in a box, with oxygen and silicon rather large, the metal atoms much smaller. The basic silicate structure, Bragg thought, would be determined by the ways the larger marbles fit together, the smaller ones could then be tucked between.

Pauling read Bragg's work and thought very highly of it. But he also understood that Bragg, a physicist, was missing something important. Chemists like Pauling thought of atoms not as separate marbles that could arrange themselves any which way but as elements that bound themselves to other elements in specific ratios.

Silicon, Pauling knew, was similar to carbon in many ways, including in its ability to bind to other atoms. Chemists call an atom's binding ability its valence. Both carbon and silicon have a basic valence of four—that is, they are most likely to bind in a stable manner to four other atoms at a time. Pauling knew as well that when carbon bound to four other atoms it did so in a way that formed a four-sided pyramid called a tetrahedron, with the carbon atom in the middle and the points at the top and the base of the pyramid being formed by the other four atoms. There was evidence that silicon formed the same shape.

Pauling also knew that oxygen atoms in crystals often formed an octahedron, a cube-shaped arrangement, with other elements. Instead of marbles in a box, Pauling began thinking of pyramids and cubes. With these basic shapes in mind, he thought next about how the blocks fit together and what determined whether they shared a side or only a point or an entire face. He began doodling pictures and then, with Ava Helens help, started folding the three-dimensional shapes out of paper and sewing them together. These paper models helped him tremendously: He could now see what fit and what did not, and try new combinations again and again.

He made his models according to a specific set of rules. The sizes of the atoms and ions involved had to match known values; the lengths and positions of bonds between ions had to correlate reasonably with what was already known from the X-ray crystallography of simpler molecules; and the positive and negative electrical charges of neighboring ions had to balance out. To these he added more rules that he discovered himself about how many blocks could fit at a corner or along an edge.

This proved something like a game. By playing the game fairly, with his imagination reined in tightly by the chemical rules he already knew about, Pauling found he could build theoretical models of silicates that seemed to match the complicated X-ray data. This was a process of working backwards, of course—modeling something likely to produce an X-ray pattern rather than using the X-ray pattern to determine a model—but by doing it in strict conformity to what was known of chemistry and physics, and abiding by the reasonable rules he had created, he found that he could come up with structures that seemed logical and, in fact, seemed to be the only ones possible.

It turned out Pauling was right. His combination of rule making and model building allowed him to crack the structures of mica, talc, topaz, and other seemingly unsolv-able minerals. The publication in 1928 of Pauling's guidelines for solving the silicates and other ionic crystals marked his first great international scientific triumph. Pauling's

This sketch from Pauling's research notes shows a proposed structure for a silicate crystal—the type of complex molecular structure impossible to solve before Pauling developed a new approach.

Rules, as they became known, simplified a complicated field, showed other researchers a new way to solve more complex crystal structures, and made Pauling at age 27 famous among crystallographers. Lawrence Bragg himself credited Pauling with developing "the cardinal principle of mineral chemistry."

This was a wonderful step forward for Pauling's career, one that might have been enough to satisfy many young chemists. But it was not enough for Pauling. He now threw himself totally into extending Heitler and London's concept

This sketch from Pauling's research notes shows a proposed structure for a silicate crystal—the type of complex molecular structure impossible to solve before Pauling developed a new approach.

of the chemical bond to all of chemistry. Pauling hammered away at the problem, but for more than two years the answers eluded him. His mind, too restless to stay on any one problem for too long, wandered to other questions. He solved more crystal structures and published an important insight into the way molecules rotate inside crystals. He and Ava Helen traveled to Europe again for several months, this time taking Linus, Jr., with them. His reputation growing, he was offered a prestigious faculty appointment at Harvard but turned it down. He did not feel at home on the East Coast, a place, he said, "where there were a lot of important people who were important just because of their birth. They had money and stature not based on their own abilities. I thought I would be a sort of second-class citizen at Harvard." Pauling was a westerner at heart, in love with a land where someone's worth was determined by what he did, not which family he was born into.

Again and again between 1928 and 1930 Pauling came back to the question of how to make the wave equation work for larger molecules. He was not alone. Heitler and London were trying to extend their work as well, although they were hindered by their limited knowledge of chemistry. A brilliant young physicist at MIT, John Slater, was also working on the problem, and in 1930 he found a clever, relatively simple way to use the wave equation to describe the shape of the patterns that electrons made in larger atoms like carbon. According to Slater's work on carbon, the areas certain electrons concentrated in—their orbitals—were not perfect spheres but stuck out from the atomic nucleus like stubby arms.

Slater's discovery spurred Pauling to make another push toward an answer in the fall of 1930. For weeks he sat at the desk of his small home study and wrote page after page of equations, trying to find an approximation, a shortcut to make the mathematics workable. His only result was frustration.

The breakthrough came late on a December night. He was working on a paradox in the carbon atom, trying to form a picture of it by adding up the wave equations for the separate electrons. Carbon has six electrons, two of which, according to the rules of quantum physics, would be paired in an orbital close to the nucleus, unavailable for any reaction. The other four would be split into different orbitals: two closer to the nucleus, the other two in orbitals that formed the stubby arms Slater had described. All four were available for forming bonds to other atoms. But because there were two types of orbitals involved, the bonds would be expected to be different as well. Chemists, however, knew that carbon most often formed four identical bonds to other atoms to form a perfect tetrahedron. How could the chemists' four identical bonds form when the physicists said that no four electrons in carbon were the same?

On that December evening, Pauling was trying to resolve the conundrum by using Heitler and London's electron exchange idea. He believed that the energy gained from the exchange when the carbon atom bound to others would be enough to break carbon's outer electrons out of their original energy levels and combine them into new forms, ones that would explain the tetrahedral binding of carbon. He called these new mixed forms hybrid orbitals.

That was fine in theory, but Pauling needed the mathematics to back him up. For months he worked on adding up Schrtidinger's wave equation for the separate electrons of carbon to make his hybrids, but nothing seemed to give the right answer. He tried shortcuts, simplifications, and approximations—all leading to dead ends and wrong answers. But on this night he found the key. In yet another attempt to simplify the mathematics, he chose to leave out a part of the wave equation that he assumed was approximately the same for all the orbitals. If something is the same on both sides of an equation, he thought, you should be able to factor it out.

So Pauling dropped a section of the mathematics—and found the shortcut he needed. With that layer of complexity gone, the equations began to fall into place. He scribbled, erased, scribbled some more, and suddenly had an answer. He had created, from the principles and equations of quantum mechanics, a description of the tetrahedral binding of carbon. Everything he had calculated added up: the bond angles, lengths, strengths. He had pushed Heitler and London's concept to a new level.

But Pauling did not stop there. Using his simplified approach, he could add more electrons to his calculations and derive more complicated molecules. "I was so excited and happy, I think I stayed up all night, making, writing out, solving the equations, which were so simple I could solve them in a few minutes," he remembered. "Solve one equation, get the answer, then solve another equation. ... I just kept getting more and more euphorious as time went by."

Two months later, Pauling sent his results to the Journal of the American Chemical Society. His paper, which he titled "The Nature of the Chemical Bond," showed chemists clearly, for the first time, how the new quantum mechanics could answer questions about the structure, magnetic properties, and bond strengths of molecules. Its publication in 1931 again brought Pauling international attention and showed the world that he was not only an extraordinary crystallographer but an accomplished chemical theorist. Later that year, he was awarded the American Chemical Society's Langmuir Prize as the best young chemist in the nation, a great honor capped by Arthur Amos Noyes's calling him "the most promising young man with whom I have ever come in contact in my many years of teaching." Caltech, realizing what a prize Pauling was, promoted him to full professor at age 30 and increased his salary.

The confidence others had in him was matched by Pauling's self-confidence. He now believed he could rebuild chemistry on a new foundation, using the wave equation text continues on page 54

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