Linus Pauling

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A page from Pauling's 1927 article in The Proceedings of the Royal Society, in which he pioneered the use of quantum mechanics to predict the size and other properties of atoms. This paper marked Pauling's debut on the international scientific stage.

visited galleries. They toured the new Deutsches Museum, with its extensive scientific exhibits, joined other students for hiking trips to the nearby Alps, and sometimes went dancing at Odeon's Casino. "I love to dance with Linus for he is such a good dancer," Ava Helen wrote in a letter home that summer. "We get along wonderfully well and do lots of little steps that other people can't do."

The only dark period came in July 1926, when Pauling received a letter from his sister Lucile saying that their mother had died. Although only 45 years old, Belle had grown increasingly frail after Pauling left for Europe, her pernicious anemia sapping her strength and causing her to have delusions. She had died, his sister wrote, in a state hospital psychiatric ward. Pauling wept as he read it.

But he buried what grief he had in work. Sommerfeld was teaching him the mathematics he needed to succeed with Schrodinger's wave equation, and he began applying this method successfully to problems. A great breakthrough came when he used wave mechanics to explain some of the basic properties, including the size, of large atoms with many electrons. This important step forward won him Sommerfeld's admiration and publication in the prestigious British journal Proceedings of the Royal Society. He next figured out how to use the new physics to predict the sizes of atoms as they existed in crystals.

These were noteworthy advances, but they still did not address Pauling's driving question about the bonds between atoms. Applying Schrodinger's mathematics to complexes of more than one atom simply did not seem to work, and Sommerfeld was not able to help him. In February 1927 he and Ava Helen left Munich to search for answers elsewhere. They first traveled to Copenhagen, where Pauling failed to catch the attention of the great Danish quantum physicist Niels Bohr. The next stop was Zurich, where he was unsuccessful in his attempts to work with Schrodinger as well. "I have rather regretted the nearlv two months spent

186 L. Pauling.

It will be observed that the function differs appreciably from zero only within a radius of the order of magnitude of the major axis of the corresponding ellipses of the old quantum theory ; namely, r 2rt0n2/Z, or £ = in, as was remarked by Schrodinger (I). In fig, 2 are given values of D as a function of

eoTTespond to r, the average vh iuc of r.

here," Pauling wrote a friend from Zurich in August 1927, "for I have been unable to get in touch with Schrodinger. I saw him about once a week, at a seminar. I tried very hard to find out what he is doing, and I offered to make any calculations interesting to him since he was not interested in my work; but without success."

Even worse, Pauling found out in Zurich that he would not be the first to apply wave mechanics to the chemical bond. Two young German acquaintances of his, Walter Heitler and Fritz London, had beaten him to it. Working closely with Schrodinger, they had found a way to use the wave equation to create a mathematical model of a simple chemical bond.

Their breakthrough was made possible by a new idea of Heisenberg's called exchange energy. This theory proposed that as two atoms approached each other the chances would increase that a negatively charged electron from one would find itself attracted to the positively charged nucleus of the other, and the same thing would happen from the other side. At a certain point, the two electrons would begin jumping back and forth between the two nuclei, creating an electron exchange that would happen billions of times per second. In a sense, the two electrons would no longer be able to tell which nucleus they belonged to.

Combining this idea with Schrodinger's wave equation, Heitler and London calculated that the attractive energy of this electron exchange would be balanced at a certain point by the repulsion of the two positively charged nuclei, creating a chemical bond with a definite length and strength. They proved their point by calculating the bond between two hydrogen atoms.

It was a great triumph, the first extension of Schrodinger's wave mechanics to the level of molecules and the first time the new physics had been used successfully to explain the nature of chemical bonds. Pauling scrutinized Heitler and London's results while he was in Zurich and talked with them often. He became convinced that they were right and recognized that his own efforts had failed because he had not used the essential idea of the electron exchange.

But there was a great deal left to do. Heitler's and London's work was just a first step. They had worked out only the simplest hydrogen molecule. There were many more problems to be solved with this new approach. Pauling determined that he would be the one to solve them.

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