By the early 1900s it was known that quantized motions could explain thermal radiation and the photoelectric effect. The first step had been taken to think of a light wave as a particle a photon. But, if light could be a particle then what should we say about the electron emitted by light in the photoelectric effect? The idea that light is a particle leads one to ask whether a particle, such as an electron, could act as a wave. The idea of treating an alectron as a wave lead to the Bohr model for the hydrogen atom. Although the Bohr model fails for any larger atoms, it did explain the observed pattern of hydrogen emission lines that had been summarized by Rydberg. Bohr assumed a planetary model, using the same type of balance of forces that keeps the earth in orbit around the sun, to exaplain the states and energies of the hydrogen atom. Of course, in the atom the Coulombic force is the attractive force while in the solar system the gravitational force is the attractive force. In both models, this attractive force is balanced by a centripedal force due to the circular trajectory. Bohr then assumed that the electron was a wave, which meant that it had an integral number of wavelengths are the circumference of the circular orbit. Finally, he used the DeBroglie relation to relate the wavelength of the orbit to the momentum. These features were all that was needed to obtain agreement with experiment.
