1. The Atom Unit 3, Presentation 1

2. History of the atom Not the history of atom, but the idea of the atom – The atom was not “discovered” until recently Original idea Ancient Greece (400 B.C.) – Proposed by lesser known scientists

3. History of Atom The looked at a beach – Made of sand If you cut sand – smaller sand What is the smallest possible piece? What is the smallest possible piece? Atomos - not to be cut Atomos - not to be cut

4. Another Greek Aristotle - Famous philosopher All substances are made of 4 elements – Fire – Hot, dry – Air - light – Earth - cool, heavy – Water - wet These are blended in different proportions to get all substances

5. Who Was Right? Greek society class based Thought to be beneath famous scientists to work with hands – Aristotle did not experiment Greeks settled disagreements by argument – Aristotle was more famous – He won His ideas carried through middle ages. – Alchemists change lead to gold

6. Who’s Next? Late 1700’s - John Dalton- England He was a teacher – summarized results of his experiments and those of other’s In Dalton’s Atomic Theory – Combined ideas of elements with that of atoms

7. Dalton’s Atomic Theory  All matter is made of tiny indivisible particles called atoms.  Atoms of the same element are identical, those of different atoms are different.  Atoms of different elements combine in whole number ratios to form compounds  Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.

8. Law of Definite Proportions Each compound has a specific ratio of elements It is a ratio by mass Water is always 8 grams of oxygen for each gram of hydrogen

9. Law of Multiple Proportions If two elements form more that one compound, the ratio of the second element that combines with 1 gram of the first element in each is a simple whole number.

10. What? Water is 8 grams of oxygen per gram of hydrogen. Hydrogen Peroxide is 16 grams of oxygen per gram of hydrogen.

11. 2

12. 8 X 2 Y 16 X8 Y +

13. Parts of Atoms J. J. Thomson - English physicist. 1897 Made a piece of equipment called a cathode ray tube. It is a vacuum tube

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

15. Thomson’s Experiment Voltage source +-

16. Thomson’s Experiment Voltage source +-

17. Thomson’s Experiment Voltage source +-

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

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

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

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

22. Thomson’s Experiment By adding an electric field + -

23. Voltage source Thomson’s Experiment n By adding an electric field + -

24. Voltage source Thomson’s Experiment n By adding an electric field + -

25. Voltage source Thomson’s Experiment n By adding an electric field + -

26. Voltage source Thomson’s Experiment n By adding an electric field + -

27. Voltage source Thomson’s Experiment n By adding an electric field + -

28. Voltage source Thomson’s Experiment n By adding an electric field he found that the moving pieces were negative + -

29. Cathode Ray Tube

30. Thomson’s Model Found the electron But he couldn’t find positive (for a while) Said the atom was like plum pudding A bunch of positive stuff, with the electrons able to be removed

31. Other pieces Proton - positively charged pieces 1840 times heavier than the electron Neutron - no charge but the same mass as a proton. Where are the pieces?

32. Rutherford’s experiment Ernest Rutherford English physicist. (1910) Believed in the plum pudding model of the atom. Wanted to see how big they are Used radioactivity Alpha particles - positively charged pieces given off by uranium Shot them at gold foil which can be made a few atoms thick

33. Lead block Uranium Gold Foil Fluorescent Screen When alpha particles strike a fluorescent screen, it glows.

34. 1.atoms positive charge is concentrated in the nucleus 2.proton (p) has opposite (+) charge of electron (-) 3.mass of p is 1840 x mass of e - (1.67 x 10 -24 g)  particle velocity ~ 1.4 x 10 7 m/s (~5% speed of light) (1908 Nobel Prize in Chemistry)

35. He Expected The alpha particles to pass through without changing direction very much Why? – The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles

36. What he expected

37. Because

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

40. What he got

41. How he explained it + Atom is mostly empty Small dense, positive piece at center n Alpha particles are deflected by nucleus if they get close enough

42. +

43. Modern View The atom is mostly empty space Two regions Nucleus- protons and neutrons Electron cloud- region where you might find an electron

44. Density and the Atom Since most of the particles went through, it was mostly empty. Because the pieces turned so much, the positive pieces were heavy. Small volume, big mass, big density This small dense positive area is the nucleus

45. atomic radius ~ 100 pm = 1 x 10 -10 m nuclear radius ~ 5 x 10 -3 pm = 5 x 10 -15 m Rutherford’s Model of the Atom “If the atom is the Houston Astrodome, then the nucleus is a marble on the 50-yard line.”