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The Development of Modern Atomic Theory In the beginning… • Democritus- In the 4th century BC, he proposed that the universe was made of tiny indivisible units. • He called these atoms, from the Greek word atomos, meaning unable to be divided or cut. • He did not have any evidence to support his theory, but nonetheless, some people believed. Dalton’s Atomic Theory • In 1808, he proposed a revised atomic theory. • According to Dalton, all atoms of a given element were exactly alike, and atoms of different elements could join to form compounds. • Theory was based on experimental evidence. • However, some parts of his work turned out to be incorrect. Thomson’s Model of the Atom • 1897- Experiments with electricity led to the discovery that atoms were NOT indivisible! • He conducted a famous cathode ray tube experiment which suggested that these rays were made of negatively charged particles that came from inside of atoms. Results from the Cathode Ray Tube Experiment • Thomson knew that a magnet deflected charges, and he reasoned that because the tube was a vacuum the beam must have come from the cathode (positively charged plate) or anode (negatively charged plate). • The direction of deflection indicated a negatively charged beam, and electrons were discovered!!!! Thomson’s Plum Pudding Model • Based on the outcome of his experiment, he proposed a new model of the atom. • He reasoned that the electrons were spread throughout the atom, just like in plum pudding, a dessert in his time. • We might use the chocolate chip cookie model to better visualize it today. Rutherford’s Model of the Atom • Ernest Rutherford proposed his model shortly after Thomson. • He found that most of the mass of an atom was concentrated at the atom’s center, or the nucleus. • To understand this conclusion, you must learn about his famous gold foil experiment. Gold Foil Experiment • A beam of positively charged alpha particles were aimed at a thin sheet of gold foil. • Because of Thomson’s model, Rutherford predicted that there would not be a large enough mass of positive charge to cause many particles to bounce back. He thought they would travel in a straight path through the foil. Gold Foil Experiment • But, the observations did not match his hypothesis. Most passed straight through, but a few were deflected by a large amount. • A few even bounced back! • He said, “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15- inch shell at a piece of tissue paper and it came back and hit you.” Gold Foil Experiment Results of Rutherford’s experiment • An atom’s positive charge in concentrated in the center of the atom, called the nucleus. • He is also credited with the discovery of the proton. • Here is his atomic model: