1. Ernest Rutherford conducted an experiment where he fired alpha particles at a thin gold foil and detected their scattering with a fluorescent screen. 2. Contrary to expectations, some alpha particles were scattered at high angles or even back in the direction of the source. 3. This unexpected result led Rutherford to conclude that atoms are mostly empty space, with a tiny, dense positively charged nucleus at their center that could deflect the alpha particles. This challenged the prevailing plum pudding model of the atom.

1. History of Atomic Structure

2. Ancient Philosophy Leucippus(490BC) and his pupil Democritus (470 – 380 BC), during one of their walks along the seashore, noted that the beach looked like one whole continuous piece from afar. A material can be broken into smaller pieces Atomos – a greek word which means indivisible

5. Dalton’s Atomic Theory In 1808, he published a book, A New System of Chemical Philosophy in which he presented the theory in detail

6. 1. All matter is composed of indestructible atoms. “ Matter, though divisible in an extreme degree, is nevertheless not infinitely divisible. That is, there must be some point beyond which we cannot go in the division of matter. The existence of these ultimate particles of matter can scarcely be doubted, though they are probably too small ever to be exhibited by microscopic improvements. I have chosen the word atom to signify these ultimate particles…” 2. The atoms of a given element are identical. They are different from the atoms of all other elements. They are unchageable. “the atoms never can be metamorphosed one into another by any power we can control” Compounds are formed by the combination of the atoms of two or more elements forming compound-atoms (what we now call molecules). The atoms combine with each other in definite ratio of small whole numbers. (Law of Definite Composition by Proust)

7. 4. Chemical reactions involve only the separation and or union of atoms. The atoms are only rearranged; none are created nor destroyed. “Chemical analysis and synthesis go no farther that the separations of particles from another, and to their reunion. No new creation or destruction of matter is within reach of chemical agency. We might as well attempt to introduce a new planet or to manipulate one already in existence, as to create or destroy a particle of hydrogen. All the changes can produce consist in separating particles that are in state of cohesion or combination, and joining those that were previously at a distance”

8. Design lang ito..

10. In England, 2000 years later, Sir William Gilbert tried similar experiments and learned that many materials when rubbed together becomes electrically charged

13. Electrostatic Attraction and Repulsion Removing Electrons from Atoms Charging by "conduction."

15. Discovery of Electrons Who:J. J. Thomson When: 1897 Where: England What: Thompson discovered that electrons were smaller particles of an atom and were negatively charged. Why:Thompson knew atoms were neutrally charged, but couldn’t find the positive particle. J. J Thomson made a piece of equipment called a cathode ray tube.

16. Discovery of electron..before Thomson Sir Humphry Davy(1778-1829) – the electrical nature of matter was revealed in his discovery that when electric current is passed through molten compounds of metals resulted in the decomposition of compounds to produce the metals. Led to the discovery of Na,K,Ca,Mg,Sr,Ba. Michael Faraday (1791-1867) – showed that mass of the element formed is proportional the quantity of electricity that was passed. Each atom of the element was interacting with a definite amount of electricity. This further suggested that electricity was interacting with definite amount of electricity. In 1891, George J. Stoney ( 1874-1911) – Irish physicist who suggested the name electron for the particle of electricity

17. Direct observation and characterization of electron Began with the work of physicist on the discharge of electricity through a vacuum tube. Heinrich Geissler (1814-1879) a German inventor, was able to device a method of producing a good vacuum in glass tubes. Julius Plucker (1801-1868), a German mathematician and physicist, sealed two metal pieces into a Geissler tube and applied a high voltage across the electrodes and observe a greenish luminescence emanating from the negative electrode, the cathode (or the negatively charged plate). The cathode ray is drawn to the positively charged plate, called the anode. Eugene Goldstein proposed that the luminescence observed by Plucker is cathode ray (later on identified as electron by Thomson).

18. Sir William Crookes (1832-1919) – an English physicist, showed that the cathode ray travelled in a straight line and that objects placed in its path cast a shadow at the opposite side of the tube (possibly a form of an Electromagnetic Radiation). Later he confirmed Plucker’s observation that the ray was deflected by a magnet in a way that indicated it to be unlike light but rather consist of negatively charged particles. Direct observation and characterization of electron

19. A Anode Cathode B C Fluorescent screen Cathode Ray Tube – N S High voltage +

20. Anode Cathode Fluorescent screen – N S High voltage +

21. Anode Cathode B Fluorescent screen No external fields – N S High voltage +

22. A Anode Cathode Fluorescent screen – N S High voltage + Magnetic field applied

23. Anode Cathode C Fluorescent screen – N S High voltage + Electric field applied

24. Anode Cathode Fluorescent screen – N S High voltage +

25. Anode Cathode B Fluorescent screen – N S High voltage + Effects of electric field and magnetic field cancel

26. – A Anode Cathode N B S C Fluorescent screen + High voltage

27. Cathode ray tube According to electromagnetic theory, a moving charged body behaves like a magnet and can interact with electric and magnetic fields though which it passes. Because cathode ray is attracted by the plate bearing positive charges and repelled by the plate bearing negative charges, it must consist of negatively charged particles. These negatively charged particles are electrons.

28. A cathode ray produced in a discharge tube travelling from the cathode (left) to the anode(right). The ray itself is invisible, but the fluorescence of a zinc sulfide coating on the glass causes it to appear green

29. The cathode ray is bent downward when the north pole of the bar magnet is brought toward it. When the polarity of the magnet is reversed, the ray bends in the opposite direction

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

32. Voltage source + -

33. Voltage source Thomson’s Experiment + -

34. Voltage source Thomson’s Experiment + -

37. Thomson’s Model Found the electron Atoms were made of small negatively charged particles. Assumed that larger part of atom is positively charged with small electrons scattered in it. Said the atom was like plum pudding

39. When a small spherical particle such as a tiny drop of water or oil is allowed to fall through the air under influence of gravity, it will reach a steady speed of fall which depends on the friction of the air and the size and weight of the sphere.

40. Oil drops are sprayed into a chamber by means of an atomizer.

41. An oil droplet is allowed to fall between two charged plates.

42. The speed of an oil droplet can be measured accurately by watching the droplet falls, and measuring the time necessary for it to pass between the crosshairs of the telescope.

43. As oil droplet falls through the air, it may acquire a positive or negative charge due to friction

45. The balanced state will be shown by the state of the particle; it will remain suspended in the mid-air

46. This provides a means for calculating the electrical force, since the force due to gravity can be measured for the speed of fall of the particle before the electrical force was applied.

47. From the strength of the electrical force, the voltage and the distance between the charged metal plates, the charge carried by the particle can be calculated.

48. He observed that the electrical charge was always a whole-number multiple of a smallest charge, which he called the unit charge.

49. The value of the unit charge is 1.60 x 10-19 coulomb.

50. If e is the unit charge, then the charge observed to be carried by a particle may be +e, -e, +2e, etc.

51. Knowing the possible values of the charge of a positive ion (+e, +2e, +3e..) it is then possible to compute its mass from the charge mass ratios. The smallest positively charged particle obtained from hydrogen gas has a charge of +e and a mass of approximately 1 amu. ---proton

52. Each electron has a charge of -1 unit, equivalent to -1.60 x 10-19

54. How is the charge of a particle measured? Charged plate (+) (–) Charged plate Atomizer Small hole Viewing microscope

55. How is the charge of a particle measured? Charged plate Oil droplets (+) Small hole (–) Charged plate Atomizer Viewing microscope

56. How is the charge of a particle measured? Charged plate Oil droplets (+) Small hole (–) Oil droplet under observation Charged plate Atomizer Viewing microscope

57. Radioactivity In 1895, German physicist Wilhelm Rontgen noticed that cathode rays caused glass and metals to emit unusual rays. These rays could not be deflected by a magnet, they could not contain charged particles as cathode rays do ---X rays. Antoine Becquerel professor of Physics in Paris began to study fluorescent properties of substances. He found that exposing thickly wrapped photographic plates to a certain uranium compounds caused them to darken, even without stimulation of cathode rays

59. Lead block Radioactive substance – a g b +

60. Lead block – +

61. Lead block Radioactive substance – a +

62. Lead block Radioactive substance – b +

63. Lead block g Radioactive substance – +

64. Lead block Radioactive substance – a g b +

65. Ernest Rutherford’s GoldFoil Experiment Who: Ernest Rutherford When: 1911 Where: England What: Conducted an experiment to isolate the positive particles in an atom. Decided that the atoms were mostly empty space, but had a dense central core. Why: He knew that atoms had positive and negative particles, but could not decide how they were arranged.

66. Ernest Rutherford (1871-1937), a British physicist and his associate Hans Geiger(1882-1945) a German physicist, studied the alpha particles emitted by radium which was isolated by Marie and Pierre Curie. Alpha particles are found to be helium atoms with their electrons removed, positively charged and mass of 2500 times that of the electron. Together with Ernest Marsden (an undergraduate student) they studied the scattering of high speed alpha particles when passed through thin metal foils.(about 2000 atoms thick) Believed in the plum pudding model of the atom. Wanted to see how big they are

67. Gold foil a–Particle emitter Slit Detecting screen Rutherford’s Experimental Design (a)

68. Gold foil a–Particle emitter Slit Detecting screen (a)

69. Gold foil a–Particle emitter Slit Detecting screen (a)

70. Florescent Screen Lead block Uranium Gold Foil

71. He Expected The alpha particles to pass through without changing direction very much Because most of the mass of the atom (positive charges) were spread. Alone they were not enough to stop the alpha particles If the Thomson model were correct, all the alpha particles, travelling at high speeds and massive, would have passed through the metal foil undeflected or only slightly deflected

72. What he expected

73. Because

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

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

76. What he got They observed that although majority of the alpha particles passed through undeflected, some were only slightly deflected, some were scattered by more than 90 degrees and a few by nearly 180 degrees or almost completely turned back

77. + How he explained it Atom consists of a very small nucleus surrounded by electrons. Rutherford estimated the radius at 10-12 to 10-13 cm compared to radius of the atom of about 10-8 cm The nucleus contains most of the mass of the atom and all of its positive charge. Alpha particles are deflected by nucleus it if they get close enough at each other

78. +

79. Ernest Rutherford’s Model (Nuclear Model of an Atom)

80. Electron Cloud Model