If I tell you when did the first contribution of this complex but interesting model took place, you would be surprised; Well, FYI the first contribution was by Democritus, approximately in 400 BC, he said that matter was composed by small particles called atoms; Then until 1804, when Mr. John Dalton came and added something to the model, that atoms of the same element are identical, that during reactions atoms are just rearranged, and that atoms combines in multiples. The next contributor is Dmitri Mendeleev, in 1869, arranged the elements known to his times in a table that today we know as periodic table of the elements; Next in 1896-9 Henri Becquerel discovered that elements have properties, he experimented with Uranium, Rutherford made an expeiment and achieved the separation of alfa and beta particles from gamma rays. In 1897 J.J. Thompson discovered the elctron after his experimentations. Between 1909 and 1911 Geiger and Rutherford identified a nucleus on the atom, this discovery was aided by the experiment of the gold foil.In 1913 Robert Milikan experimented and did some research to identify the mass of an electron, in the same year, J.J. Thompson discovered the positively charges particles on the nucleus and discovered the isotopes and H. Mosely determined the atomic mass of the elements, and also reorganized the periodic table.
In 1919 Mr. Rutherford, proposed the existence of neutrons on the nucleus, as he confirmed protons.
The last significant contribution, was the one by James Chadwick in the 1930's he confirmed the nucleus as the center of the particles.
The following image illustrates the models as the were imgined by each of the scientists.
References
Document about atomic model
A brief history of Atomic Theory development
