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Lecture 1.2 – Development of Atomic Theory What was Dalton’s contribution to atomic theory? I. Dalton’s Model of the Atom Draw this! What was Thomson’s contribution to atomic theory? I. J. J. Thomson • First evidence for subatomic particles. • Made discovery in 1896. + - + - - + - + + - - + - + + - II. J. J. Thomson’s Experiment • Cathode-ray tube II. J.J. Thomson’s Experiment • Put a battery to the cathode tube and saw a beam form. • The beam contained subatomic particles. • Positive magnet next to beam = bent towards magnet. • Negative magnet next to beam = bent away from magnet. Table Work – What subatomic particle did he discover? III. Thomson’s Discovery • Thomson discovered the electron! • He discovered that atoms must contain some negatively charged particles called electrons. • Tells us nothing about the location of the electron. IV. Plum Pudding Model • Thompson proposed that electrons were balls of negative charge floating in a sea of positive charges. Draw this! What was Rutherford’s contribution to atomic theory? I. Ernest Rutherford • Disproved Plum Pudding Model with Gold Foil Experiment • Used alpha particles, which are small, positively charged particles. II. Rutherford’s Experiment • Shot positively charged alpha particles at gold foil • Most particles went through foil. • Small amount of the particles were deflected • Smaller amount were bounced back Rutherford noted: “It was almost as if you fired a 15-inch shell into a piece of tissue paper and it came back and hit you.” III. What Rutherford’s Data Meant • Most alpha particles went straight through = atom is mostly empty space. • Some alpha particles deflected = they came close to other electrons. • Small amount bounced back = hitting a very small, but very dense positively charged nucleus. IV. Important Point #1 • The atom is mostly empty space. V. Important Point #2 • The atom has very small, but very dense nucleus. • Alpha particles that bounced back were coming into contact with nucleus. Draw this!