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Democritus • Greek philosopher • Came up with the idea of an atom • Named it “atomos ” meaning a piece of matter that was “not able to be divided” 440 BCE John Dalton 1803 Developed the Atomic theory: • All substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed. • Atoms of the same element are alike, atoms of different elements are different • Atoms join together to make new substances. JJ Thomson • Discovered negatively charged particles inside the atom now called electrons • Proposed the plumpudding model 1897 Ernest Rutherford • Tested Thomson’s theory using Gold-foil experiment • Proposed that in the center of the atom is a dense, positively charged part called the nucleus 1909-1911 Niels Bohr • Proposed that electrons move around the nucleus in certain paths called energy levels • Electrons can jump from one level to the next 1913 Schrodinger and Heisenberg 20th century • Electrons do not travel in definite paths • There are regions where electrons are likely to be found called electron clouds