2. The Origins of the Atomic Theory Pisgah High School M. Jones Rev. 02/20/09

3. The Development of the Atomic Theory Democritus and Dalton Crookes, Roentgen, Becquerel: evidence for subatomic particles Thomson, CRT’s and the electron Millikan - “Oil Drop Experiment” Rutherford -“Gold Foil Experiment”

4. Democritus Greek philosopher ~ 400 BC Limit to “smallness” Disagreed with Aristotle and the concept of “infinity” All matter consists of tiny, indestructible particles called atoms Atomos – indestructible

5. John Dalton Atomic Theory English scientist Studied the properties of gases “Reinvented” the idea of atoms Published in 1803

6. 1. Elements are composed of tiny, discrete, particles called atoms. Dalton’s atomic theory - 1803

7. 2. Atoms are indivisible and indestructible and do not change their identity during reactions. Dalton’s atomic theory - 1803

8. 3. Atoms of the same element are identical in mass and chemical and physical properties. Atoms of different elements are different. Dalton’s atomic theory - 1803

9. 4. Atoms combine to form compounds in simple, whole- number ratios, as in H 2 O. Dalton’s atomic theory - 1803 Supports Proust’s Law of Definite Proportions

10. 5. The same atoms can combine in different ratios to make two or more compounds, as in FeO and Fe 2 O 3. Dalton’s atomic theory - 1803 Law of Multiple Proportions

11. Dalton’s Atomic Theory 1.Atoms are tiny, discrete particles 2.Atoms are indestructible 3.Atoms of the same element have the same mass and properties 4.Atoms combine in simple whole- number ratios 5.Atoms in different ratios produce different compounds.

12. Dalton’s Atomic Theory 1.Atoms are tiny, discrete particles 2.Atoms are indestructible 3.Atoms of the same element have the same mass and properties 4.Atoms combine in simple whole- number ratios 5.Atoms in different ratios produce different compounds.

13. Evidence for subatomic particles During the 19 th century many discoveries were made that were later shown to involve subatomic particles. Cathode rays, canal rays, X-rays, and alpha, beta and gamma rays were investigated.

14. Evidence for subatomic particles Each helped advance the foundation of what would later become the atomic theory. People like Crookes, Goldstein, Roentgen, Becquerel, Pierre and Marie Currie, Thomson, Millikan, Rutherford and others all paved the way for us to be able to talk about atoms.

15. William Crookes Used spectroscopy to discover thallium and used vacuums to measure its mass. Invented the radiometer. Improved vacuum systems -- used by Edison to make light bulbs.

16. William Crookes Developed what was called the Crookes’ Tube … which is what we now call a cathode ray tube. … which is what we now call a cathode ray tube.

17. William Crookes Used the cathode ray tube to to study electric fields in a vacuum and discovered rays, … which were called “cathode rays” by Goldstein, since they came from the cathode, or negative electrode.

18. William Crookes The shadow of the Maltese cross indicates that cathode rays travel in straight lines and can be stopped by a solid object.

19. William Crookes He found that the cathode rays could be deflected by a magnet. This suggested that the cathode rays might be a stream of electrically charged particles.

20. Cathode Ray Tube High voltage CathodeAnode Direction of cathode rays +

21. Cathode Ray Tube High voltage CathodeAnode Direction of cathode rays + Magnet

22. Cathode Ray Tube High voltage CathodeAnode + Used by J. J. Thomson … to discover the electron.

23. J.J. Thomson and Cathode Rays Attracted to positive electrode Thought might be atoms Had same charge to mass ratio regardless of metal in the cathode The particle was much less massive than the lightest element – H Particle must be common to all matter, a subatomic particle

24. In 1897 J. J. Thomson discovered the electron. In 1897 J. J. Thomson found that cathode rays are a basic building block of matter.

25. The term “electron” actually comes from George Stoney’s term for the “minimum electrical charge”. After the discovery of the electron, it was assumed that this particle was the carrier of the minimum electrical charge and so the particle was called an “electron”.

26. J. J. Thomson Even though Crookes and others observed and characterized cathode rays, Thomson is credited with the discovery of the electron because he recognized that it was a fundamental particle of nature as well as a sub-atomic particle.

27. J. J. Thomson Measured the charge to mass ratio, and found … … that if this “minimum charge” was equal to the charge on a hydrogen ion, then the mass of the electron would be 1/1837 th the mass of a hydrogen atom.

28. J. J. Thomson If that were the case, then the electron would be much smaller than the smallest atom, … showing for the first time that matter is made up of particles smaller than atoms. Thomson tried to measure the fundamental charge on the electron.

29. Robert A. Millikan Robert A. Millikan, an American physicist, set out to determine the charge on an electron. From 1909 through 1910, he performed what is now called the “Oil Drop Experiment”.

30. High Voltage Robert A. Millikan Telescope Cast iron pot Atomizer

31. High Voltage Robert A. Millikan Telescope Cast iron pot Atomizer Oil Drop Parallel charged plates

32. Robert A. Millikan Radiation stripped electrons from the oil droplets. The charged droplets fell between two electrically charged plates. By adjusting the voltage, he could change the rate of fall or rise of a single oil drop. After observing hundreds of drops, he calculated the charge on a single electron.

33. Robert A. Millikan Charges on drops are multiples of 1.602 x 10 -19 coulombs.

34. Robert A. Millikan The fundamental charge on an electron is 1.602 x 10 -19 coulombs. With J. J. Thomson’s charge to mass ratio, and Millikan’s charge on the electron, we are able to compute the mass of an electron: 9.109 x 10 -28 gram

35. Robert A. Millikan 9.109 x 10 -28 gram 0.0000000000000000000000000000919 Mass of an electron: g

36. Properties of Radiation Alpha, Beta and Gamma Investigate properties of Radiation.

37. Mica window (fragile) Wire (+ side of circuit) Metal shield (- side) Low pressure Ar gas Counter 2435 Geiger-Mueller Tube

38. Rays leave the source Some hit the GM tube Most do nothing One ray may cause a discharge… Source and the detector clicks Geiger-Mueller Tube

39. Ernest Rutherford Named alpha, beta and gamma rays and determined their properties. The leading authority on radioactivity. Alpha particles Beta particles Gamma rays   

40. Three kinds of radioactivity Alpha particles An unstable nucleus splits to form a more stable nucleus an an alpha particle. An alpha particle is the nucleus of a helium atom. Two protons and two neutrons. Has a positive charge.

41. Three kinds of radioactivity Beta particles Ejected from the nucleus when a neutron decays. A beta particle is identical to an electron Has a negative charge.

42. Three kinds of radioactivity Gamma rays Emitted by the nucleus as the nucleus becomes more stable Electromagnetic energy Short wavelengths and high energy.

43. Gamma rays have short wavelengths … and high energies. Increasing energy

44. Three kinds of radioactivity -helium nuclei -electrons -high energy electromagnetic energy - similar to light, but higher in energy. Alpha particles Beta particles Gamma rays

45. Alpha, Beta, Gamma Radioactive Source - - - - - - - - - + + + +    Electrically charged plates

46. Alpha, Beta, Gamma Radioactive Source  Paper Aluminum foil Lead

47. Alpha, Beta, Gamma Radioactive Source   Paper Aluminum foil Lead

48. Alpha, Beta, Gamma Radioactive Source    Paper Aluminum foil Lead

49. Radioactivity … … the natural decay of unstable atoms. … can be detected by photographic film or a Geiger counter. … is “ionizing radiation”. Causes cell damage and mutations – cancer. … is protected against by shielding and distance.

50. The Gold Foil Experiment 1909. E. Rutherford instructs Hans Geiger and his assistant, Ernest Marsden, to investigate the scattering of alpha particles by thin metal films.

51. The Gold Foil Experiment Scattering of alpha particles … Earlier, Rutherford had noticed “fuzzy shadows” formed as alpha particles struck very thin materials.

52. The Gold Foil Experiment Scattering of alpha particles … Light beamAlpha particles

53. The Gold Foil Experiment Top View Side View

54. The Gold Foil Experiment Alpha particle source Gold foilFluorescent detector ZnS All of this was in a vacuum chamber.

55. The Gold Foil Experiment Most of the  particles went… …straight through the gold foil, undeflected. The gold is mostly “empty space.”

56. The Gold Foil Experiment Most alpha particles were undeflected. A few were bent through large angles – a very few came back toward source. Rutherford said it was like shooting at tissue paper and the shell bouncing back and hitting you.

57. Alpha Particles Alpha particles are helium nuclei. + + Two protons and two neutrons. The alpha particle has a positive charge.

58. Gold Foil Experiment: Results +  source Small, dense, positively charged nucleus

59. Gold Foil Experiment: Review +  source The positive  particles are repelled by the nucleus.

60. Rutherford’s Nuclear Atom Alpha particles were repelled by… … a small, dense, positively charged nucleus. Almost all the mass of an atom is in the nucleus. Electrons are located outside the nucleus. Published results in 1911.

61. Rutherford and the Proton 1917 – 1924: Rutherford experimented with radioactivity and alpha particles. Bombarded the lighter elements with alpha particles. N +   O + H Transmutation occurred.

62. Rutherford and the Proton Rutherford was the first person to cause one element to change into another element. N +   O + H A nitrogen atom was changed into an oxygen atom.

63. Rutherford and the Proton Hydrogen gas was also produced. N +   O + H A nitrogen atom was changed into an oxygen atom.

64. Rutherford and the Proton N +   O + H The hydrogen atom must consist of the simplest part of the nucleus of an atom. The nucleus of a hydrogen atom must be a proton.

65. Rutherford and the Proton N +   O + H 7 protons 2 protons 1 proton 8 protons 9 protons

66. Chadwick and the Neutron Worked with Rutherford on alpha bombardment from 1919. Then later on the search for a neutral particle in the nucleus. Both disagreed with the current theory of extra protons and electrons in the nucleus.

67. Chadwick and the Neutron Particles can be detected by their ability to ionize air, but neutral particles did not ionize air. He did experiments (1932) which showed an undetected radiation knocking protons out of paraffin. The radiation consisted of neutrons.

68. No, not really the end. This is just the beginning of the “nuclear age”. The End

69. Helium nucleus (2 p + 2 n). Blocked by 2.5 cm of air, 3-4 sheets of paper or by skin. Very dangerous when released inside the body. Relatively massive and slow. Alpha Particles

70. High speed electrons from nucleus. Much faster than alphas. Blocked by metals or plastic. Dangerous to cells. Much less massive. ~  Beta Particles

71. Electromagnetic energy, not particles. Like light but invisible, much higher energy and shorter wavelengths. Travel at the speed of light. Gamma Rays

72. Greater penetrating power. Blocked by many inches of lead or many feet of concrete. Can easily pass through your body, and can damage cells. Gamma Rays

73. Mica window (fragile) Wire (+ side of circuit) Metal shield (- side) Low pressure Ar gas Counter 2435 Geiger-Mueller Tube

74. Rays leave the source Some hit the GM tube Most do nothing One ray may cause a discharge… Source and the detector clicks Geiger-Mueller Tube

75. GM Tube Filled with low pressure argon gas About 1% efficiency About 1 in 100 rays causes an electric spark between the case and the wire Each spark registers as a count or click on the counter