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Ancient Greece

~400 B. C. Ancient Greece. Democritus: . “All matter is made of tiny, unbreakable particles; atomos ”. Aristotle: “All matter is made of 4 elements; earth, air, fire and water”. Aristotelian viewpoint triumphs for 2000 years…. 2000 years later….

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Ancient Greece

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  1. ~400 B. C. Ancient Greece Democritus: . “All matter is made of tiny, unbreakable particles; atomos” Aristotle: “All matter is made of 4 elements; earth, air, fire and water” Aristotelian viewpoint triumphs for 2000 years…

  2. 2000 years later… John Dalton, a British Teacher in the late 1700’s, combined ideas of elements with that of atoms and developed Dalton’s Atomic Theory

  3. Dalton’s Atomic Theory: • All matter is made of tiny indivisible particles called atoms. • Atoms of the same element are identical. • Atoms combine to form compounds in simple whole number ratios. (example: electrolysis) • Chemical reactions involve the rearrangement of atoms; no matter is created or destroyed.

  4. 100 years later, another classy brit enters the scene: J.J. Thompson:English Physicist (1856 – 1940)Nobel Prize in Physics 1906 Famous for: • Cathode Ray Experiments • “Discovery” of proton, neutron and electron • ‘Plum Pudding’ Model of the Atom

  5. Voltage source Thomson’s Experiment - + • Passing an electric current makes a beam appear to move from the negative to the positive end

  6. Voltage source Thomson’s Experiment + - • By adding an electric field he found that the moving pieces were negative (electrons!)

  7. This, and Thompson’s other experiments showed that atoms had… positive, (protons) negative, (electrons) andneutral(neutrons) …pieces inside them. (but where in the atom were they?)

  8. electrons + - - + positively charged “pudding” + - - - + + - - - + + + - - - + + + + + + - - - - - - + - + - + - + + + conduction of electricity Thompson suggested a“Plum Pudding” Model of the atom: It explained conduction of electricity:

  9. Thompson’s Model of the Atom: A bunch of positive stuff, with the electrons able to be removed “Plum Pudding” Model; a bunch of positive ‘pudding’, with removable negative ‘plums’ (electrons)

  10. Plum Pudding gets spilled by: Ernest Rutherford:New Zealand Physicist (1871-1937) Nobel Prize in Physics 1908 Famous for: • Gold Foil Experiment • “Discovery” of the nucleus • Nuclear Model of the Atom

  11. Rutherford’s experiment Took positive charges (alpha particles; radioactive particles that are emitted by Uranium) and fired them at Gold Atoms His surprise: Some of them bounced back.

  12. How it worked: When the alpha particles hit a florescent screen, the screen glows. So, he could keep track of where the positive charges ended up after he fired them at some gold foil.

  13. What he expected What he saw:

  14. He Expected • The alpha particles to pass through without changing direction very much • Because • The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles…

  15. Because, he thought the mass was evenly distributed in the atom

  16. + How he explained what he saw: • Atom is mostly empty • Small dense, positive piece at center • Alpha particles are deflected by it if they get close enough

  17. Result of Rutherford’s Gold Foil Experiment: “Discovery” of the Nucleus Rutherford’s Scattering Experiment metal foil alpha particles Ernest Rutherford 1871-1937 film loop what he expected: ricocheting alpha particles! what he got:

  18. ++ + - - + + ++ - - + - + ++ ++ ++ ++ 10-8 cm 10-13 cm Goodbye, Plum Pudding; • Thomson’s Atom: • diffuse mass and charge • + alpha particles would have passed through Hello, Nuclear Model • Rutherford’s Atom: • concentrated mass and positive charge at the nucleus • electrons roam empty space around the nucleus • some + alpha particles repelled

  19. Niels Bohr:Danish Physicist (1885 – 1962)Nobel Prize in Physics 1906 Famous for: • Planetary Model of the Atom • “Father of” Quantum Theory / Mechanics “Electrons have specific energy levels, …like planets have specific orbits.”

  20. Bohr said: Electrons can only be at certain, specific energy levels (distances from the nucleus) n = 1 n = 2 n = 3 n = the primary quantum number (distance from nucleus

  21. Electrons are found in ‘shells’ = ‘energy levels’ can hold 18 electrons can hold 8 electrons can hold 2 electrons

  22. Valence Shell: The outermost shell that contains electrons. Valence Electron: An electron in an atom’s valence shell

  23. Bohr Model Diagrams…show energy levels (shells) and the electrons in them.nucleus is not shown. 1st shell (n = 1) gets 2 e- Inner shells fill first; 2nd shell (n = 2) gets 8 e- 3rd shell (n = 3) gets 8 e- …before 4th shell starts to fill

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