Pauling, under the direction of major professor Roscoe Dickinson, had done his doctoral work on molecular structure: determining the architecture of molecules, the positions of atoms, the angles and distances between them, using a new technology called x-ray crystallography. It would form an important part of his approach to the chemical bond.

Almost all solids exist in crystalline forms, in which the atoms are arranged in repeating three-dimensional patterns. Some crystals, including those in many metals, are too small to be seen with the naked eye. Others, from table salt to emeralds, quartz to rock candy, can grow very large. But regardless of the crystal’s size, it was impossible to "see" the positions of the underlying atoms.

Until 1912, when Max Theodor Felix von Laue, a German physicist, discovered a roundabout way of seeing the unseeable. He did it by shooting a beam of x-rays at crystals and then analyzing the way the atoms in the crystal reflected the x-rays. The resulting "diffraction pattern" could be seen by developing a piece of photographic film placed near the crystal. By analyzing the pattern created by the scattered x-rays — a process requiring very complex mathematical calculations — researchers could painstakingly work out, at least for simple crystals, the distances and angles between the atoms that comprised them. Once the basic crystal unit involved more than a handful of atoms, however, the patterns became too complex to analyze directly.

Researchers had guessed at molecular structures for decades. But without any way to verify the guesses, they remained just that. X-ray crystallography was a fantastic tool. Caltech was one of the first places to use it in a comprehensive research program. And again, Pauling would lead the way.