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Democritus proposed that there was a limit to how much matter could be subdivided and that there existed an ultimate, indivisible particle called an atom. No experiments were done at this stage.
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Isaac Newton proposed a mechanical universe with small solid masses in motion. He also produced the three laws of motion.
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Dalton did many experiments and found that elements always combined in fixed ratios. He also proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass.
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G.J. Stoney proposed that electricity was made of discrete negative particles he called electrons.
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Thomson and other scientists all experimented with passing electricity through gases at low pressures in sealed glass tubes,
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Max Planck used the idea of quanta (discrete units of energy) to explain hot glowing matter.
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Nagaoka postulated a "Saturnian" model of the atom with flat rings of electrons revolving around a positively charged particle.
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Milikan's oil drop experiment determined the charge and mass of an electron.
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He established that the nucleus was: very dense, very small and positively charged. He also assumed that the electrons were located outside the nucleus.
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He wrote “The atomic number of an element is equal to the number of protons in the nucleus".
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The neutron was discovered.
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Developed an explanation of atomic structure that underlies regularities of the periodic table of elements. His atomic model had atoms built up of successive orbital shells of electrons.