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Democritus stated that all matter was composed of atoms and could no longer be divided.He acknowledged the idea that all atoms were composed of the same primary matter, but with different sizes, shapes, and weights.
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Aristotle felt that regardless of the number of times you cut a form of matter in half, you would always have a smaller piece of that matter. He believed in the four elements of air, earth, water and fire.
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John Dalton developed the first coherent atomic theory. He stated that all matter is composed of atoms, that all atoms of an element are identical, during a chemical reaction atoms are neither made nor destroyed, and atoms always combine in whole number multiples.
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Developed a better vacuum pump that allowed him to produce cathode-ray tubes with a smaller residual gas pressure.
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Ernest Rutherford proposed the nuclear atom as the result of the gold-foil experiment.
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Thomson proposed that an atom was composed of a spherical ball of positive charge with "corpuscles" of negative charge imbedded in it. The corpuscles would later become known as electrons.
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The Curies discovered another radioactive element in pitchblende. They named it `polonium’ in honor of Marie Curie’s native homeland.
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Robert Millikan determined the unit charge of the electron .
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Henry Moseley discovered that the energy of x-rays emitted by the elements increased in a linear fashion with each successive element in the periodic table.
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Moseley proposed that the relationship was a function of the positive charge on the nucleus. This rearranged the periodic table by using the atomic number instead of atomic mass to represent the progression of the elements.
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Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus.
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