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400 BC Democritus - empty world proposal. 1803 - John Dalton - proposed that atoms can’t be destroyed , just rearranged. Dalton formulated modern atomic theory. -all matter is composed of atoms -atoms cannot be made or destroyed -all atoms of the same element are identical -different elements have different atoms -chemical reactions occur when atoms are rearranged -compounds are formed when atoms form consistent elements -the law of conservation of mass, the law of constant composition, the law of multiple proportions 1886- Eugene Goldstein Observed a cathode ray tube and found a rays traveling in the opposite direction of the cathode ray he concluded that they were composed of positive particles 1897 through 1898- JJ Thomson credited with the discovery of the electron. Plum pudding model Of the atom. the model had negatively charged electrons evenly distributed throughout a sphere of positively charged material 1909-1911 Ernest Rutherford- discovery of nucleus Gold foil experiment , atom was mainly empty space 1909-1911 Millikan did oil drop, found the charge of the electron - found electrons mass w thomson’s experiments 1913 Neils Bohr added on Rutherford’s model of the atom by adding fixed orbitals for electrons. Bohr is credited with the atomic model 1976 James Chadwick discovered neutron. Used radiation tracking to discover the neutron.