Ultimately, the chemical behavior of an atom depends upon the number of protons in the nucleus. This number determines the number of electrons moving outside the nucleus. The number and energies of the electrons become of key interest to chemists.
The electron was characterized by J.J. Thomson in 1897. Background information on Millikan's experiment.
With Ernest Rutherford's [local] discovery of the nucleus and the resulting nuclear model of the atom, problems [local] with the role of electrons in the atom were apparent.
The connection between the electrons and the light emitted by excited atoms was a very critical step in the development of our modern atom. See:
A nuclear model of the atom was proposed by Niels Bohr [local]. Early in the twentieth century there were a number of problems that impacted together and greatly changed the way people viewed energy and matter. Light had been viewed by scientists as a wave but two important problems and these answers caused a change in the concept of light. These problems were the Black Body Radiation Problem [local] and the Photoelectric Effect [local].
The changes in the concept of light proposed by Planck and Einstein began what is today termed the Quantum Theory.
