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Atomic Theory The Who’s Who of Atoms Democritus • ~460 BCE. • Defined atom as the “smallest bit of matter. • 100 years later, Aristotle dismissed his idea as “worthless.” John Dalton • 1805 • Said matter consisted of elementary lumpy particles • First Atomic Theory • 5 Parts to his model: Dalton’s Atomic Model 1. All matter is made of tiny particles called atoms. 2. All atoms of one element are alike. 3. Atoms of one element are different from atoms of other elements. 4. Compounds are formed when atoms from different elements combine. 5. Chemical reactions involve rearrangement of atoms, not changes in atoms themselves. J.J. Thomson • 1897 • Discovered the electron with his Cathode Ray Tube Experiment • Came up with the “Plum Pudding Model” of atom Cathode Ray Tube Experiment Plum Pudding Model “Real” Plum Pudding Max Planck • 1900 • When you vibrate atoms enough (heat it until it glows), you only get energy in whole intervals: energy packets called “quanta.” Ernest Rutherford • 1911 • Discovered the nucleus with his Gold Foil Experiment • Nucleus: the small, dense, positivelycharged center of the atom • In 1919, he identified “protons” Rutherford’s Atom Robert A. Millikan • 1916 • Found charge and mass of electron with his Oil Drop Experiment Oil Drop Experiment Niels Bohr • 1912 • Developed planetary model of the atom • Discovered that electrons move around the nucleus at set distances • Called these “electron orbitals.” Electron Orbitals “Bohr Model” of the Atom James Chadwick • 1932 • Discovered the neutron So Then We Have This! Let’s Memorize These Guys! • It’s the least we can do to commemorate all their hard work! • Quiz on Friday! • 5 minute break? Then SUBATOMIC PARTICLES!