With a fresh Ph.D. in his pocket, Niels Bohr journeyed from Denmark to England in 1911 and came under the influence of two of the giants of atomic science, J. J. Thomson in Cambridge, and Ernest Rutherford in Manchester. Back in Copenhagen the following year, Bohr began to work out the details of a quantum mechanical model of the atom. His 1913 paper, describing what we now call the Bohr atom, represented the first successful application of quantum ideas to the structure of matter. (Application to radiation had come earlier.)
Bohr drew upon three main threads of earlier work that, to that time, had not been successfully woven together. First was the basic quantum idea of Planck and Einstein that radiation is emitted and absorbed only in discrete energy packets, with the energy of a single packet and its frequency being related by the equation E = hf. (Interestingly, Bohr accepted Planck’s quantized energy transfer but not Einstein’s quantized radiation.) Second was the highly developed empirical science of spectroscopy, particularly the simple regularities of the hydrogen spectrum and the so-called Ritz combination principle (which revealed simple numerical relationships among the frequencies of radiation from particular atoms). Third was the new physical picture of the atom advanced by Rutherford—a heavy positively charged nucleus of small dimensions surrounded by electrons. Bohr recognized that in some way Planck’s quantum constant had to play a role in the mechanics of the atom, for only then could the motion of the electrons and the processes of emission and absorption of radiation be drawn together in a unified theory. With what one colleague has called daring conservatism, Bohr abandoned only as much of classical mechanics as necessary, holding firm to those classical ideas that still seemed to work within the atom.