Greek Idea

1. Greek Idea • Democritus and Leucippus • Matter is made up of solid indivisible particles • John Dalton’s model of the atom – tiny hard sphere

2. Just How Small Is an Atom? • Think of cutting a piece of lead into smaller and smaller pieces • How far can it be cut? • An atom is the smallest particle of an element that retains the properties of that element • Atoms-very small • still observable with proper instruments

3. Parts of Atoms • J. J. Thomson - English physicist. 1897 • Made a piece of equipment called a cathode ray tube. • It is a vacuum tube - all the air has been pumped out.

4. Voltage source Thomson’s Experiment - + Vacuum tube Metal Disks

5. Voltage source Thomson’s Experiment - +

6. Voltage source Thomson’s Experiment - +

7. Voltage source Thomson’s Experiment - +

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

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

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

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

12. Voltage source Thomson’s Experiment • By adding a magnetic field

13. Voltage source Thomson’s Experiment + - • By adding a magnetic field

14. Voltage source Thomson’s Experiment + - • By adding a magnetic field

15. Voltage source Thomson’s Experiment + - • By adding an electric field

16. Voltage source Thomson’s Experiment + - • By adding a magnetic field

17. Voltage source Thomson’s Experiment + - • By adding a magnetic field

18. Voltage source Thomson’s Experiment + - • By adding a magnetic field he found that the moving pieces were negative

19. Thomson’s Model • Found the electron • Couldn’t find positive (for a while) • Said the atom was like plum pudding • Negatively charged electrons embedded in a ball of positive charge

20. J. J. Thomson’s Model

21. Rutherford’s experiment • Ernest Rutherford English physicist (1908) • Believed in the plum pudding model of the atom. • Wanted to see how big the atoms are • Alpha particles – fast moving positively charged pieces given off by uranium • Shot them at gold foil which can be made a few atoms thick

22. Rutherford’s experiment • When the alpha particles hit a florescent screen, it glows. • Here’s what it looked like

23. Florescent Screen Lead block Uranium Gold Foil

24. 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

25. What he expected

26. Because, he thought the positive chargeswereevenly distributed in the atom

27. Because, he thought the positive charges wereevenly distributed in the atom

28. What he got

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

30. +

31. Density and the Atom • Since most of the particles went through, it was mostly empty. • The region of positive charge must have a large mass, or else the fast-moving alpha particles would have knocked it out of the way (but instead the direction of travel of the alpha particles was reversed). • Small volume, big mass, big density • This small dense positive area is the nucleus

32. Ernest Rutherford’s Model • Electrons would surround THE NUCLEUS • “Nuclear model”