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Greek philosopher, Democritus, was a central figure in the development of the atomic theory of the universe. He theorized that all material bodies are made up of indivisibly small “atoms.”
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Aristotle did not believe in the atomic theory and he taught so otherwise. He thought that all materials on Earth were not made of atoms, but of the four elements, Earth, Fire, Water, and Air. He believed all substances were made of small amounts of these four elements of matter
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Lavoisier found that mass is conserved in a chemical reaction. The Law of Conservation of Mass.
chemed.chem.purdue.edu -
Dalton proposed the theory of chemical combination; elements consist of indivisible small particles (atoms), all atoms of the same element are identical/different elements have different types of atoms, and atoms can neither be created nor destroyed.
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Mendeleev found when all the known chemical elements were arranged in order of increasing atomic weight, the resulting table displayed a recurring pattern, or periodicity, of properties within groups of elements.
britannica.com -
Thomson's experiments with cathode-ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. Thomson proposed the plum pudding model of the atom, which had negatively charged electrons embedded within a positively charged "soup."
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Millikan began a series of experiments to determine the electric charge carried by a single electron.
Nobelprize.org -
Rutherford's experiments showed that when alpha particles are fired into gas atoms, a few are violently deflected, which implies a dense, positively charged central region containing most of the atomic mass.
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Bohr proposed a theory for the hydrogen atom, electrons move around a nucleus, but only in prescribed orbits, and If electrons jump to a lower-energy orbit, the difference is sent out as radiation.
NobelPrize.org -
Chadwick proved the existence of neutrons, bombarding elements with neutrons can succeed in penetrating and splitting nuclei, generating an enormous amount of energy.
Atomicheritage.org