During the latter part of the nineteenth century and the early years of the twentieth century numerous experiments were performed that probed matter on a very fine scale. The observations required explanation, and thus began the development of atomic theory. Experiments using the scattering of charged particles, as well as electromagnetic radiation, demonstrated that the atom has structure. Rutherford's experiment established that the mass of an atom is concentrated in a very small nucleus, while its volume and its physical and chemical properties are determined by a comparatively loose surrounding structure of electrons. Other experiments, such as the conduction of electricity through rarified gases, showed the existence of free electrons. Milliken's oil drop experiment provided direct confirmation that the electron's charge-to-mass ratio is a constant, and that the charge on a droplet can be measured in integral amounts. Numerous explanations were advanced for these observations, some with success and some without. One of the more impressive results from the Lorentzian theory of the electron was the determination of the classical radius of the electron.

Electron structure of atoms

To explain his results, Rutherford advanced a planetary model for the atom, that of planetary electrons orbiting a solar nucleus. This model, however, encountered serious difficulties. The laws of classical mechanics predict that the electron would emit electromagnetic radiation while orbiting the nucleus. As a result of the radiation emission the electron would lose energy and would gradually spiral inward, collapsing onto the nucleus. The classical model predicts that all atoms are unstable. In addition, as the electron spirals inward the emission would increase in frequency as the orbit got smaller and faster, and one would see a smear in frequency of electromagnetic radiation. Experiments showed, however, that light is emitted only at certain discrete frequencies.