When the Atom Went Quantum: Bohr's Revolutionary Atomic Theory Turns 100

Article excerpt

Before Niels Bohr, atoms baffled science's brightest brains.

For millennia, atoms had been phantoms, widely suspected to exist but remaining stubbornly invisible--though not indivisible, as their name (Greek for "uncuttable") originally implied. By the start of the 20th century, physicists knew that atoms had electrically charged parts; the favorite model envisioned blobs of positively charged pudding studded with negatively charged plums (actually, electrons). That image was challenged, though, when Ernest Rutherford showed in 1911 that the positive pudding was all crammed into a massive dense core, or nucleus, surrounded at a distance by the electron plums (SN: 5/7/11, p. 30).

But Rutherford's atom baffled everyone even more, as the laws of physics prohibited the arrangement that he described. Opposite charges attract each other relentlessly; electrons should spiral into the atom's positive nucleus in less than a millisecond. (Even if they didn't, their mutually repulsive negative charges would blast them out of their orbits.) Yet somehow atoms housed negative and positive charges happily.

Into this paradox stepped a great Dane, a genius conditioned by his culture to embrace conflicting ideas and learn from them. A century ago, Niels Bohr married the old standard physics with the new quantum theory, giving birth to the modern model of the atom's structure.

Bohr's atom did more than simply reconcile theory with experiment. Bohr figured out the basics of how atoms hook up to make molecules. He explained the mysterious repetition of properties displayed by the periodic table of the chemical elements. And most consequential of all, he established the fundamental role of quantum physics in describing the underlying reality of the universe.

Even though the technical details of Bohr's model turned out to be wrong, he had grasped the essential idea for understanding atoms: abandoning common sense in favor of the crazy rules of quantum theory. Bohr saw more deeply than others of his time that embracing quantum physics was the key to unlocking nature's hidden truths. While quantum confusions drove other physicists to despair, Bohr pursued the path into the yellow quantum wood. When two roads diverged, he traveled both but remained one traveler, insisting that knowing reality meant accepting the truth of mutually incompatible viewpoints.


In the decades following his description of

the atom, Bohr served as guide and interpreter for the world's physicists as they explored the strange new quantum world. As the physicist J. Robert Oppenheimer observed, in the development of modern quantum physics, "the deeply creative and subtle and critical spirit of Niels Bohr guided, restrained, deepened, and finally transmuted the enterprise."

Father of the atom

Bohr's role in that enterprise began in 1913 with a series of three papers that became the foundation for the future of atomic science.

Bohr "gave the first firm and lasting direction toward an understanding of atomic structure and atomic dynamics," physicist Abraham Pais wrote in his biography of Bohr, Niels Bohr's Times (1991). "In that sense he may be considered the father of the atom."

Like most fathers, Bohr was proud of his offspring. But he was not blind to its faults. He knew from the beginning that his atom model was too simple to capture all of reality's complexities. He was certain, though, that explaining the atom required quantum physics. "That, of course, was the key to Bohr's great invention," says science historian John Heilbron, of the University of California, Berkeley.

Bohr had foreseen the need for quantum theory when investigating the electron theory of metals for his 1911 doctoral dissertation. He found that electrons carrying current and those bound to atoms behaved in different ways, at odds with the ordinary mechanical laws of classical physics. …