Democritus first proposed the idea that all matter was made up of tiny indivisible particles called atomos
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Antoine Lavoisier determined the Law of Conservation of Matter by extensive experimentation
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Joseph Proust determined the law of constant composition (compounds always contain the same proportion, by mass, of elements)
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John Dalton formulated an atomic theory of matter with 4 postulates:
1. Elements are composted of small particles called atoms.
2. All atoms of a given element are identical. *
3. Atoms are neither created nor destroyed in chemical reactions.
4. Compounds always contain the same proportion of elements.
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J.J. Thompson performed a CRT experiment and determined that the ray was composed of negative particles, call these electrons, determined the mass-charge ratio for an electron, came up with the "plum pudding" model of the atom
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Robert Millikan performed an "oil-drop" experiment, determined the charge of one electron and using the known mass-charge ratio, was able to calculate the mass on one electron (9.11x10^-28g)
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The Curies (Marie and Peirre) discovered two other radioactive elements, polonium and radium
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Ernest Rutherford performed the gold foil experiment, determined that atoms contained a very small, positively charged core at the center, called this core the nucleus, came up with the nuclear model of the atom (with electrons orbiting around the nucleus)
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Henry Moseley found that all atoms of a given element contained the same number of protons in the nucleus
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Niels Bohr developed the quantized orbit model of the atom, electrons can only reside in certain energy levels
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Erwin Schroedinger expanded the idea that electrons behaved as particles AND waves, this developed the quantum mechanical model of the atom
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James Chadwick discovered the neutron