Who Thought of the Atomic Theory? What did they think? http://simple.wikipedia.org/wiki/Atomic_theory
Democitus came up with it first, a greek philosopher. He believed that if you kept cutting something in half and making it get smaller and smaller, eventually, it could not be cut any smaller and still be there. He called those small pieces Atoms, and stated that they could never be changed or destroyed.
An english scientist, John Dalton, revisited and revised Democritus's theory, rewriting it. He stated; All matter is formed from Atoms, Atoms cannot be divided or seen, Atoms of the same element have the same type and mass, Atoms that make compounds have set proportions, and chemical reactions respond to atoms taking part of the chemical reaction.
Thomson's model of an Atom. http://simple.wikipedia.org/wiki/Atomic_theory
Sir Joseph John Thomson, after the Cathode ray experiment, said that emitted rays were negative charges, since they were on a positive charged pole. He knew that all atoms were electrically neutral, but said that they should have the same quantity of the negative or positive charge.
Rutherford's model of the Atom. http://simple.wikipedia.org/wiki/Atomic_theory
In 1910, a Physicist from New Zealand, known as Ernest Rutherford, had a suggestion, or idea, that the positive part of an atom was at it's central, the Nucleus, with the Electrons surrounding it. By his stage, the main elements of said atoms were discovered, along with the new knowledge that they could occur in Isotopes. Even though this was a good model, modern day science has made more progress since then, and made it easier to understand.
Bohr's model of the Atom. http://en.wikipedia.org/wiki/Bohr_model
Niels Bohr in 1913 came up with the Bohr model, showed the atom as a small, positively charged nucleus, with Electrons that circle it on the outside, with the electrons rotating around the atom like the planets rotate around the sun in the solar system. However, instead of a gravitational pull like in the solar system, there is a Electronic pull.
Schrodinger's model of the Atom. http://www.abcte.org/files/previews/chemistry/s1_p6.html
Edwin Schrodinger, an Australian physicist, took Bohr's model of the atom and enhanced it further. Instead of Bohr and predicting the exact location an an Electron, he calculates the odds of said electron location, where it would end up. Said model was showed as a nucleus surrounded by a cloud of electrons. Hence, where the cloud appears the thickest is where the electron is most likely at, and vice versa. This also introduced the concept of sub-energy levels.
Chadwick, an english scientist, realized that an atom had a neutron. This discovery helped with the uranium fissure 235, because neutrons don't need to overcome a certain barrier. He stated that the neutron was in the nucleus too, with neither a positive nor negative charge.