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History of Atomic Theory Democritus: Atom meaning “unable to divide”. (440 BCE) Dalton’s Atomic Theory: (1803) o All substances are made of atoms. o Atoms cannot be created, divided, or destroyed. o Atoms of the same element are exactly alike. Atoms of different elements are different. o Atoms join together with other atoms to make new substances. JJ Thomson: “Plum pudding” Model (1897) o Discovered the electron. Rutherford: “Gold foil” experiment (1911) o Atom has a positive center (nucleus). o Atoms are made up of mostly empty space. Niels Bohr: Electrons are found in energy levels (1913). o Compared electrons, orbiting the nucleus in energy levels, to planets orbiting around the sun. Modern Model: Electrons are believed to behave more like waves on a vibrating string than like particles. o The best scientists can do is calculate the chance of finding an electron in a certain place within an atom.