2. Atoms and their Structure

3. History of the Atom n Original idea Ancient Greece (400 B.C..) n Democritus* and Leucippus: Greek philosophers

4. Democritus Reasoned n Looked at beach n Once had rocks n Break down rocks? n Eventually sand n Smallest possible piece? n Atomos - indivisible

5. Another Greek n Aristotle - Famous philosopher n All substances are made of 4 “humors” n Fire – Hot, vaporizes earth n Air – light, vapor (old term – “ether”) n Earth - cool, heavy, rocks, trees n Water – wet, liquid but also ice, vapor n Blend these in different proportions to get all substances

6. Who Was Right? n Greek society was slave based n Beneath the famous to work with hands n Did not experiment n Greeks settled disagreements by argument n Aristotle was more famous n He won n His ideas carried through middle ages.

7. Alchemists Had two dreams: n 1: live forever (Philosopher’s Stone) n 2: turn “base”metals into gold n Lots of trial and error experiments n Accomplishments? n Metallurgy – Metal from ores; Alloys n natural pharmacy and medicine

8. Who’s Next? n Late 1700’s - John Dalton- England n Teacher- summarized results of his experiments and those of other’s n In Dalton’s Atomic Theory n Combined ideas of elements with that of atoms

9. 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. (Familiar idea?)

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

11. Law of Multiple Proportions n 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.

12. What?! n Water is 8 grams of oxygen per gram of hydrogen. n Hydrogen Peroxide is 16 grams of oxygen per gram of hydrogen. n 16 to 8 is a 2 to 1 ratio n True because you have to add a whole atom, you can’t add a piece of an atom.

13. Parts of Atoms n J. J. Thomson - English physicist. 1897 n Worked with a piece of equipment called a Crookes tube. n It is a vacuum tube - all the air has been pumped out and a small amount of another gas has been let in.

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 + - cathode anode

22. Voltage source Thomson’s Experiment n 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. But Look Going the Other Direction! Faint particles were seen Deflected oppositely to the electrons Called them “protons”

30. Thomson’s Model n Found the electron (-) n Later found the proton (+) n Said the atom was like plum pudding n (Let’s say a banana- nut muffin) n A bunch of positive “stuff”, with the electrons able to be removed

31. Other pieces n Proton (Thomson) - positively charged pieces 1840 times heavier than the electron n Where are the pieces?

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

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

34. Lead block Uranium Gold Foil Florescent Screen

35. He Expected n The alpha particles to pass through without changing direction very much n Because: n The positive charges were spread out evenly. Alone they were not enough to stop the heavy 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 + n Atom is mostly empty n Small dense, positive piece at center n Alpha particles are deflected by it if they get close enough

42. +

43. Introducing the Rutherford-Bohr Model Working under Rutherford, Neils Bohr decided the electrons were scattered around the nucleus in shells or energy levels Similar to the planets orbiting around the sun: Solar System or Planetary Model

44. Also in the Nucleus n Neutron - 1930’s(James Chadwick) n - no charge but the same mass as a proton. Very faint stream, unaffected by plates, therefore no charge n Note the late date for this discovery!

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

46. Density and the Atom n Since most of the particles went through, it was mostly empty. n Because the particles occasionally deflected a great deal, the positive pieces were heavy. n Small volume+big mass=big density n This small dense positive area is the nucleus

47. Subatomic particles Electron Proton Neutron NameSymbolCharge Relative mass Actual mass (g) e-e- p+p+ n0n0 +1 0 1/1840 1 1 9.11 x 10 -28 1.67 x 10 -24

48. Structure of the Atom n There are two regions n The nucleus »With protons and neutrons »Positive charge »Almost all the mass n Electron cloud- Most of the volume of an atom »The region where the electron can be found

49. Size of an atom n Atoms are small. n Measured in picometers, 10 -12 meters n Hydrogen atom, 32 pm radius n Nucleus tiny compared to atom n IF the atom was the size of a stadium, the nucleus would be the size of a marble. n Radius of the nucleus near 10 -15 m. n Density near 10 14 g/cm

50. Counting the Pieces n *Atomic Number = number of protons n # of protons determines kind of atom n Identifies the element* n Is equal to the number of electrons in the neutral atom (Why?)

51. How Heavy is the Atom? Mass Number = the number of protons + neutrons All the things with mass

52. Something’s Wrong! n Atoms identical to hydrogen were discovered to definitely have three different Mass Numbers! n masses of 1, 2, or 3 were identified

53. Isotopes n Dalton was wrong n Atoms of the same element can have different numbers of neutrons n different mass numbers* n called *isotopes

54. Nuclear Notation n Contain the symbol of the element, the mass number and the atomic number

55. Nuclear Notation n Contains the symbol of the element, the mass number and the atomic number X Mass number Atomic number

56. But Why Different from P.T.? n Nuclear notation allows for a quick subtraction n Gives the number of neutrons in this isotopes

57. Nuclear Notation n Find the –number of protons –number of neutrons –number of electrons –Atomic number –Mass Number F 19 9

58. n Find n Find the –number –number of protons of neutrons of electrons –Atomic –Atomic number –Mass –Mass Number Br 80 35

59. n if an element has an atomic number of 34 and a mass number of 78 what is the –number of protons –number of neutrons –number of electrons –Complete symbol

60. n if an element has 91 protons and 140 neutrons what is the –Atomic number –Mass number –number of electrons –Complete symbol

61. n if an element has 78 electrons and 117 neutrons what is the –Atomic number –Mass number –number of protons –Complete symbol

62. Naming Isotopes n Put the mass number after the name of the element n carbon- 12 n carbon -14 n uranium-235 Why don’t we use the atomic number to identify the isotope?

63. Atomic Mass n How heavy is an atom of oxygen? n There are different kinds of oxygen atoms. n More concerned with average atomic mass. n Based on abundance of each element in nature. n Don’t use grams because the numbers would be too small

64. Measuring Atomic Mass n Unit is the Atomic Mass Unit (amu) n One amu is equal to one twelfth the mass of a carbon-12 atom. (Originally, the hydrogen atom was assigned a mass of one amu. Why hydrogen?) n Each isotope has its own atomic mass we need the average from percent abundance.

65. Calculating averages n You have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks? n Total mass = 4 x 50 + 1 x 60 = 260 g n Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 n Average mass = 4 x 50 + 1 x 60 = 260 g 55 5

66. Calculating averages n Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 5 n Average mass =.8 x 50 +.2 x 60 n 80% of the rocks were 50 grams n 20% of the rocks were 60 grams n Average = % as decimal x mass + % as decimal x mass + % as decimal x mass +

67. Averaging Grades n Test grade = 100% n Quiz grade = 10%! n 100 + 10 = 110 n 110/2 = 55% n Is this fair? Why? n Weighted Average

68. Let’s Pretend… Suppose there are 2 types of lithium atoms: lithium-6 and lithium -7

69. Let’s Pretend… Suppose there are 2 types of lithium atoms: lithium-6 and lithium –7 90% of the lithium in the world is Lithium-7

70. Let’s Pretend… Suppose there are 2 types of lithium atoms: lithium-6 and lithium –7 90% of the lithium in the world is Lithium-7 10% of the lithium in the world is Lithium-6

71. Let’s Pretend… Suppose there are 2 types of lithium atoms: lithium-6 and lithium –7 90% of the lithium in the world is Lithium-7 10% of the lithium in the world is Lithium-6 How do I find the average mass of lithium atoms?

72. Weighted Average! % means out of 100— % means out of 100— Then out of 100 Li atoms we have 90(7) + 10(6) = 6.9 100 100 weighted average of the masses of the isotopes. weighted average of the masses of the isotopes. Why do we not divide by 2 now?

73. Atomic Mass n Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest has a mass of 64.93 amu. n (did you get about 63.55?)

74. Atomic Mass n Magnesium has three isotopes. 78.99% magnesium 24 with a mass of 23.9850 amu, 10.00% magnesium 25 with a mass of 24.9858 amu, and the rest magnesium 25 with a mass of 25.9826 amu. What is the atomic mass of magnesium? n If not told otherwise, the mass of the isotope is the mass number in amu n (did you get somewhere around 24.3?)

75. Atomic Mass n is not a whole number because it is an average. is the number with the decimal numbers on the periodic table. is the number with the decimal numbers on the periodic table.