1922 Niels Bohr
Contribution to atomic theory: Developed an explanation of atomic structure that underlies regularities of the periodic table of elements. His atomic model had atoms built up of sucessive orbital shells of electrons.
Description of how he discovered: Niels Bohr was one of Rutherford's students. Rutherford's model thought that electrons just moved around the nucleus randomly. However, Bohr changed Rutherford's model due to newer discoveries about how the energy of an atom changes when it absorbs or emits light. When light is absorbed, the electrons move to higher energy levels, or orbitals, around the nucleus. Light is emitted when these electrons lose energy and step down in levels. Because of these new discoveries about energy levels of electrons, Bohr proposed a new model for the atom.

Greek era Democritus "by convention bitter, by convention sweet, but in reality atoms and void"
Discovery: 400 BC
Contribution to atomic theory: 1.All matter consists of invisible particles called atoms. 2. Atoms are indestructible. 3. Atoms are solid but invisible. 4. Atoms are homogenous. 5. Atoms differ in size, shape, mass, position, and arrangement.
Description of how he discovered: Democritus didn't actually do any experiments; he used theoretical reasoning to conclude that matter could only be divided up to a certain point.

1803 John Dalton
Contribution to atomic theory: Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass.
Description of how he discovered: He performed a series of experiments on mixtures of gases to determine what effect properties of the individual gases had on the properties of the mixture as a whole. While trying to explain the results of those experiments, Dalton developed the hypothesis that the sizes of the particles making up different gases must be different.

1897 J.J. Thomson Contribution to atomic theory: a CRT to experimentally determine the charge to mass ratio (e/m) of an electron =1.759 x 10 8 coulombs/gram.