Chpt 4 atomic theory 101404


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Chpt 4 atomic theory 101404

  1. 1. The Origins of theAtomic Theory Pisgah High School Rev. 1/30/02 M . Jones 1/5/04 10/14/04
  2. 2. The Development of the Atomic Theory• Democritus and Dalton: atomic theory• Crookes, Roentgen, Becquerel, Curies: early evidence for subatomic particles• Thomson: CRT’s and the electron• Millikan: “Oil Drop Experiment”• Rutherford: “Gold Foil Experiment”• Chadwick: Neutron
  3. 3. Democritus• Greek philosopher ~ 300 BC• Limit to “smallness”• All matter consists of tiny, indestructible particles called atoms• Atomos – indestructible• Aristotle and infinity
  4. 4. John Dalton First serious atomic theory• English scientist• Studied the properties of gases• “Reinvented” the idea of atoms• Published in 1803
  5. 5. Dalton’s atomic theory - 1803 1. Elements are composed of tiny, discrete, particles called atoms.
  6. 6. Dalton’s atomic theory - 1803 2. Atoms are indivisible and indestructible and do not change their identity during reactions.
  7. 7. Dalton’s atomic theory - 1803 3. Atoms of the same element are identical in mass and chemical and physical properties. Atoms of different elements are different.
  8. 8. Dalton’s atomic theory - 1803 4. Atoms combine to form compounds in simple, whole-number ratios. Law of Definite Proportions
  9. 9. Dalton’s atomic theory - 1803 5. Atoms combine in different ratios to make two or more compounds. Law of Multiple Proportions
  10. 10. Dalton’s Atomic Theory1. Atoms are tiny, discrete particles2. Atoms are indestructible3. Atoms of the same element have the same mass and properties4. Atoms combine in simple whole- number ratios5. Atoms in different ratios produce different compounds.
  11. 11. Dalton’s Atomic Theory1. Atoms are tiny, discrete particles2. Atoms are indestructible3. Atoms of the same element have the same mass and properties4. Atoms combine in simple whole- number ratios5. Atoms in different ratios produce different compounds.
  12. 12. Evidence for subatomic particlesDuring the 19th century manydiscoveries were made that were latershown to involve subatomic particles. Cathode rays, canal rays, X-rays, and then alpha, beta and gamma rays were discovered and studied.
  13. 13. Evidence for subatomic particles People like Crookes, Goldstein, Roentgen, Becquerel, Pierre and Marie Currie, Thomson, Millikan, Rutherfordand others all paved the way for us to be able to talk about atoms.Each helped advance the foundationof what would later become theatomic theory.
  14. 14. William CrookesStudied spectroscopy anddiscovered thallium.Used vacuums to measure the mass. Invented the radiometer. Made better vacuums. Techniques which were used by Edison to make light bulbs.
  15. 15. William Crookes Developed what was called theCrookes’ Tube … which is what we now call a cathode ray tube.
  16. 16. William CrookesUsed the cathode ray tube to tostudy electric fields in vacuum anddiscovered rays, …which were called “cathode rays”by Goldstein, since they camefrom the cathode, or negativeelectrode.
  17. 17. Cathode Ray TubeNear-vacuum inside the glass tube Direction of cathode rays Cathode Anode High voltage +
  18. 18. William CrookesHe found that the cathoderays could be deflected bya magnet. This suggested that the cathode rays might be a stream of charged particles.
  19. 19. Cathode Ray Tube Direction of cathode rays Cathode Anode High voltage +
  20. 20. Cathode Ray Tube Magnet Direction of cathode rays Cathode Anode High voltage +
  21. 21. Wilhelm RoentgenUsed cathode rays to study theluminescence the rays created incertain chemicals. To observe the faint glow, he surrounded the cathode ray tube with black cardboard.
  22. 22. Wilhelm RoentgenDiscovered that some bariumplatinocyanide was glowingeven though none of thecathode rays could reach itbecause they were blocked bythe cardboard.
  23. 23. Wilhelm RoentgenAn invisible radiation was comingfrom the cathode ray tube andpassing through the cardboard. He called them X-rays.Now we know X-rays as high energyelectromagnetic radiation caused bythe sudden stopping of electrons.
  24. 24. Henri BecquerelWanted to see if fluorescentsubstances produced X-rays.Out in the sun, he put a crystal of afluorescent chemical onphotographic film which waswrapped in black paper.The chemical was uranium sulfate.
  25. 25. Henri BecquerelRadiation penetrated the black paper.Because, when developed, the filmwas fogged. Must be X-rays.
  26. 26. Henri BecquerelAfter several cloudy days with theuranium sulfate and wrapped filmsafely in a drawer,he processedthe film to seeif there wasany residualfluorescence.
  27. 27. Henri BecquerelThis was what he found. The fogged area was even larger.The foggingdid notinvolve eithersunlight orfluorescence.
  28. 28. Henri BecquerelHe studied the radiationfrom the uranium Found it similarcompound. to X-rays. Could penetrate materials and ionize air.
  29. 29. Henri BecquerelThe radiation was not X-rays. It was a new kind of radiation,But it behaved from a new X-rays. Marie Curie named it radioactivity.
  30. 30. Henri BecquerelHe also found that radioactivitycould be deflected by a magnet. Could be steams of tiny charged particles.In 1900 hedecided theywere electrons.
  31. 31. Three kinds of radioactivity• Alpha particles − α• Beta particles − β• Gamma rays − γ These were named by Ernest Rutherford.
  32. 32. Three kinds of radioactivity• Alpha particles - helium nuclei• Beta particles - electrons• Gamma rays - high energy electromagnetic energy
  33. 33. Radioactivity …… the natural decay of unstable atoms.… can be detected by photographic film or a Geiger counter.… is “ionizing radiation”. Causes cells damage and mutations – cancer.… is protected against by shielding and distance.
  34. 34. Properties of Radiation Look at shielding, speed, hazards and mass.Alpha, Beta and Gamma
  35. 35. Properties of RadiationIdentityRelative MassRelative SpeedHazardsShielding Alpha, Beta and Gamma
  36. 36. Alpha Particles Helium nucleus (2 p + 2 n). Relatively massive and slow. Very dangerous when inside the body. 100% absorbed. Blocked by 2.5 cm of air, 3-4 sheets of paper or by skin.
  37. 37. Beta Particles Electrons that come from the decay of neutrons in the nucleus Much less massive. Much faster than alphas. Dangerous to cells. Blocked by metals or plastic.
  38. 38. Gamma Rays Electromagnetic energy, not particles. Like light but invisible, much higher energy and shorter wavelengths. Travel at the speed of light. Have no mass.
  39. 39. Gamma RaysCan easily pass through your body, and can damage cells.Greater penetrating power. Blocked by many inches of lead or many feet of concrete.
  40. 40. Alpha, Beta, Gamma Electrically charged plates + + + + + + + + β γ - - - - - - - - - αRadioactiveSource
  41. 41. Alpha, Beta, Gamma Paper Lead α Aluminum foilRadioactive or woodSource
  42. 42. Alpha, Beta, Gamma Paper Lead β α Aluminum foilRadioactive or woodSource
  43. 43. Alpha, Beta, Gamma Paper Lead β γ α Aluminum foilRadioactive or woodSource
  44. 44. Cathode Ray TubeIt was also used by J. J. Thomson Cathode Anode High voltage +
  45. 45. J. J. Thomson• Cathode rays - cathode ray tube• Attracted to positive electrode• Thought they might be atoms• Had same charge to mass ratio regardless of metal in the cathode• Particle must be common to all matter, a subatomic particle
  46. 46. That particle was called the … TheThe electron electron The Electron The electronThe electron Discovered in 1897 The electron By J. J. Thompson
  47. 47. The term “electron” actually comesfrom George Stoney’s term for the“minimum electrical charge”.After the discovery of the electron,it was assumed that this particlewas the carrier of the minimumelectrical charge and so the particlewas called an “electron”.
  48. 48. J. J. ThomsonEven though Crookes and othersobserved and characterizedcathode rays, Thomson iscredited with the discovery of theelectron because he recognizedthat it was a fundamental particleof nature -- a sub-atomic particle.
  49. 49. J. J. ThomsonMeasured the charge to massratio, and found … … that if this “minimum charge” was equal to the charge on a hydrogen ion, then the mass of the electron would be /1837th the 1 mass of a hydrogen atom.
  50. 50. 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 thefundamental charge on the electron.
  51. 51. Robert A. MillikanRobert A. Millikan, an Americanphysicist, set out to determine thecharge on an electron. From 1909 through 1910, he performed what is now called the “Oil Drop Experiment”.
  52. 52. Robert A. MillikanRadiation stripped electrons from the Atomizeroil droplets. The charged droplets fellbetween two electrically chargedplates. By adjusting the voltage, he Oil Drop Highcould change the rate of fall or rise of Voltage Telescopea single oil drop. After observing Cast iron pothundreds of drops,iron pot Cast he calculated thecharge on a single electron.
  53. 53. Robert A. Millikan Atomizer High Oil DropVoltage Telescope Cast iron pot
  54. 54. Robert A. MillikanCharges on drops are multiples of 1.602 x 10-19 coulombs.
  55. 55. Robert A. MillikanThe fundamental charge on anelectron is 1.602 x 10-19 coulombs.With J. J. Thomson’s charge to massratio, and Millikan’s charge on theelectron, we are able to compute themass of an electron: 9.1 x 10-28 gram
  56. 56. Ernest Rutherford• Authority on radioactivity.• Named alpha, beta and gamma rays.• Geiger and Marsden do a series of alpha scattering experiments. (1909)• Most alpha particles undeflected. Few underwent large changes – some came back toward source.• Similar to shooting at tissue paper
  57. 57. The Gold Foil Experiment Top View Side View
  58. 58. The Gold Foil Experiment Gold foil Fluorescent detector ZnS Alpha particle sourceAll of this was in avacuum chamber.
  59. 59. The Gold Foil Experiment Most of the α particles went… …straight through the gold foil, undeflected.The gold is mostly “empty space.”
  60. 60. Alpha Particles Alpha particles are helium nuclei. Two protons + and + two neutrons.The alpha particle is positively charged.
  61. 61. Gold Foil Experiment: Resultsα source + Small, dense, positively charged nucleus of gold
  62. 62. Gold Foil Experiment: Reviewα source The positive α + particles are repelled by the nucleus.
  63. 63. Rutherford’s Nuclear AtomAlpha 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 thenucleus. Published results in 1911.
  64. 64. Rutherford and the Proton1917 – 1924: Rutherford experimentedwith radioactivity and protons. Bombarded the lighter elements with alpha particles. Some protons wereknocked loose - transmutation occurred.The first person to cause a change fromone element to another. N+α  O+H
  65. 65. Rutherford and the Proton N+α  O+H7 protons 1 proton 2 protons 8 protons 9 protons 9 protons
  66. 66. Chadwick and the NeutronWorked with Rutherford on alphabombardment from 1919. Then later on the search for a neutral particle in the nucleus.Both disagreed with the currenttheory of extra protons and electronsin the nucleus.
  67. 67. Chadwick and the Neutron Particles can be detected by their ability to ionize air, but neutral particles did not ionize air.He repeated experiments (1932)which showed an undetectedradiation knocking protons out ofparaffin. The radiation consisted of neutrons.
  68. 68. Many more scientists contributed to the development and refinement of the atomic theory.