The Birth of the Bohr Model

We follow Niels Bohr from his 1911 dissertation on the electron theory of metals to his 1913 trilogy on the constitution of atoms and molecules. The dissertation shows that Bohr was thoroughly familiar with the early work of predominantly German physicists on quantum theory and that he suspected that the behavior of bound rather than free electrons called for new laws of physics. During postdoctoral work with Rutherford in Manchester, Bohr learned about the alpha-scattering experiments by Geiger and Marsden that led Rutherford to suggest that an atom consists of a nucleus containing most of its mass with a cloud of electrons swirling around it. Bohr tried to infer the atomic structure in more detail from these and further alpha-scattering experiments. Bohr’s models are in the tradition of British atomic modeling of J.J. Thomson and others but Bohr also borrowed from Planck the notion that energy is proportional to frequency. These early ideas have been preserved in the so-called Manchester memorandum, a set of notes Bohr prepared for Rutherford before returning to Copenhagen in July 1912. In this memorandum, Bohr only considered the ground state of an atom and focused on chemical rather than spectroscopic phenomena. He first started thinking about excited states when he encountered models similar to his own by another British model builder, Nicholson. His interest shifted from chemistry to spectroscopy when a Danish colleague, Hansen, alerted him to the Balmer formula. Within a month of first laying eyes on Balmer’s formula, Bohr submitted the first installment of his trilogy, which contains his famous model of the hydrogen atom. In the following months he completed the trilogy, dealing with more complicated atoms and molecules and presenting results directly coming out of the research recorded in the Manchester memorandum.