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In 350BC, Aristotle, a Greek phlisopher, discounts the Atomic Theory as he thinks more thought based than scientific based. He believed all substances were composed of water, earth, wind, and fire. http://en.wikipedia.org/wiki/Aristotle
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400BC, Lucippus, a Greek philosopher, speculates a natural philpsophy called Atomism. The notion that nature consists of two fundamental principles: atom and void, and that all matter is made up of small, indivisible praticles. http://www.egs.edu/library/leucippus/
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Democritus created the first atomic model that was simply a round sphere. It contained neither protons, neutrons, nor electrons.
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In 465BC, Democritus adopts Atomism from his mentor Leucippus and it becomes the Atomic Theory. The Atomic Theory states that "The universe is composed of two elements: the atoms and the void in which they exist and move.". Democritus also added that atoms were made of the same material and that they were capable of joining together. http://profmokeur.ca/chemistry/history_of_the_atom.htm
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John Dalton, the first in over 2000 years to add to the Atomic Theory. He developed the first useful atomic theory around 1803. He said atoms can be neither created nor destroyed. In addition, he also said compounds are formed by a combination of two or more different kinds of atoms. Further, John Dalton also indicated that all atoms of a given element are identical in mass and properties. John had the same model as Democritus. http://profmokeur.ca/chemistry/history_of_the_atom.htm
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J.J. Thomson changed the view of atoms by discovering the electron. He assumed that there were smaller particles in atoms that are negatively and positively charged. He also said that an atom consists of a sphere of a positive charge with negative charged in it. http://profmokeur.ca/chemistry/history_of_the_atom.htm
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Around 1904, J.J. Thomson proposed his model of the atom calling it the "Plum Pudding Model". http://profmokeur.ca/chemistry/history_of_the_atom.htm
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During 1911, Ernest Rutherford discovered the nucleus of the atom by shooting alpha particles at gold foil. The particles were expected to shoot straight through the foil but suprisingly deflected in various directions including right back at him. His theory was that something must have caused the particles to deflected and he concluded that the atoms contained a nucleus that deflected them. http://profmokeur.ca/chemistry/history_of_the_atom.htm
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Sometime after Ernest's gold foil experiment he created his own atomic model that shows the electrons (red) orbit around the nucleus in the center (black). http://profmokeur.ca/chemistry/history_of_the_atom.htm
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A few years after Ernest Rutherford presented his atomic theory. A Danish physicist, Niels Bohr elaborated his work by proposing that electrons could orbit the nucleus without losing energy and that the electrons also orbit in fixed positions around the nucleus. Basically, then electrons were like planets and the nucleus was a sun. http://www.rsc.org/chemsoc/timeline/pages/1913.html
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Niels Bohr's planets and sun like atomic model. Niels Bohr also discovered that the nucleus in an atom is positively charged. http://www.ask.com/question/what-did-niels-bohr-discover-about-atoms
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Around 1925, a few scientists including Ernest Rutherford and Werner Heisenberg came up with the Electron Cloud Model. The model shows that electrons do not orbit in fixed locations, but appear in the most likely spots they would. http://www.universetoday.com/38282/electron-cloud-model/
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Around 1927 Werner Heisenberg came up with the Uncertainty Principle. This priciple exhibits that electrons do not travel in fixed orbits, but in unsure places. http://the-history-of-the-atom.wikispaces.com/Werner+Heisenberg
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Around 1932, James Chadwick, a physicist, hypothosized that if an atom has a negative and positive charge, it must have a neutral charge too. He discovered the neutron. The vital part of an atom that balances it. http://www.tutorvista.com/science/chadwick-atomic-model
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James Chadwick's atomic model that shows the protons (blue), neutrons (yellow), and neutrons (red). http://www.tutorvista.com/science/chadwick-atomic-model
