Uncovering the Bohr Atomic Model

As with the early and selective uses of Special Relativity, it is difficult to disentangle  the initial Bohr atomic model from all the phenomenologically-based interests and uses at the time it first became available.  Special Relativity was first used to enhance the possibilities of a purely electromagnetic origin for mass.  Similarly, Bohr’s earliest model was quickly brought to play in every spectroscopic problem.  And the Bohr model began to be modified immediately – the first such modification being Bohr’s adjustments to account for the variable finite masses of the nucleus of different atoms and isotopes.  Observational confirmations of the model came in thick and fast mingled with skepticism about the model’s proposed structure.  The skepticism consisted mostly of misgivings about how electrons remained stable and at constant energies in their inmost sets of orbits.  The radiating and absorption at quantized levels of quantized energies that fit known spectral series was generally seen as the main value of the model despite its classically inexplicable nature. 

Bohr’s model is often characterized as “semi-classical,” and this characterization is perhaps too perfect to really be useful.  For example, like Thomson Scattering, it was worked out at a time when quantum methods were only beginning to be used.  Like Thomson scattering, it works as a phenomenological method for some situations.  However, unlike most “semi-classical” models, such as the partly semi-classical Klein-Nishina photon scattering formula, it was so rapidly modified that it is used in its simple, original form only in textbook reconstructions of what the simple and instructive initial model does describe well.  So in that sense, the Bohr model is more the first completely quantum-based model than it is really a semi-classical model.

For example, the Bohr model’s representation of the energy levels of Hydrogen electrons in 1913 was elaborated by Sommerfeld by 1915 and these 1913 representations can be set as the beginning of a series of better quantum-based representations ( Bohr, Sommerfeld, Dirac, QED) – but still, Bohr’s 1913 model is the beginning of these quantum-derived energy level theories, so in that sense, Bohr’s earliest model is no more classical than Dirac’s or that of Quantum Electrodynamics.

If we look at a marvelous book by two astrophysicists in 1922, The Origin of Spectra, we can see right away how quickly the Bohr model became part of the standard toolkit of physics and how quickly it entered into a steady flow of refinements that led eventually to Quantum Electrodynamics.  Here is a page from The Origin of Spectra that shows the movements in electron energy levels as reflected in emission spectra.  The Lyman series for Hydrogen was predicted by the Bohr model in 1913 and confirmed in 1915:

The next page from The Origin of Spectra shows Sommerfeld’s additional quantized aspects of electron energy with his elliptical orbits introduced in 1915 and thereafter an integral part of the Bohr model: