hile Pauling was at Caltech, a small group of European theorists led by Niels Bohr was rethinking how the world is made. Bohr's ideas seemed, at first, bizarre. He believed in the theo ries of Max Planck, a German physicist who in 1901 had proposed that forms of energy such as heat and light were not continuous and smooth, as Newton had thought, but grainy and discontinuous, made of discrete bits he called quanta. Evidence in favor of Planck's quanta grew throughout the first decade of the 20th century until 1913 when Bohr, then 28 years old, proposed that the atom itself was a quantum system.
In Bohr's theory, the atom consisted of electrons circling the nucleus, but only at specific distances from the nucleus, orbits with diameters restricted by quantum rules. Add a quantum of energy to the atom and a Bohr electron would "jump" from an orbit closer to the nucleus to one farther away. Then, falling back to a more stable orbit, it would release a quantum of energy, sometimes in the form of visible light.
Bohr's theory offered an explanation of why elements gave off characteristic wavelengths of light when they were heated, patterns of light that could be passed through a prism and studied, and which formed a sort of spectral fingerprint for each element. Because each element had its own unique set of electrons, each when heated would be expected to throw off a distinctive pattern of light as the electrons fell back into their original orbits.
By the early 1920s, Bohr, refining his ideas through work with the German physicist Arnold Sommerfeld, was drawing pictures of atoms that looked like gorgeous geometric flowers, their intricate petaling formed of elongated, elliptical, interpenetrating electron orbits that were all carefully related to the elements' spectral patterns. During the years Pauling was at Caltech, those complex atomic constructions, with their pulsing, wheeling, harmonious electron orbit and chordlike sets of light-lines, appeared to represent, as Sommerfeld said, "the true music of the spheres."
But there were still questions. How could electrons disappear from one orbit and reappear in another without existing anywhere in between, having
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