2. Ch. 2 The History of the Atom Z:\video streaming\Matter_Is_A nything_That_Takes_U p_Space.asf

3. Table of Contents  Ancient Philosophers & Alchemists Ancient Philosophers & Alchemists  Democritus & Aristotle Democritus & Aristotle  Dalton’s Theory Dalton’s Theory  Thomson’s Electron Thomson’s Electron  Rutherford’s Nucleus Rutherford’s Nucleus  Goldstein’s Protons Goldstein’s Protons  Chadwick’s Neutron Chadwick’s Neutron  Atomic Symbols Atomic Symbols Back to Table of Contents

4. The Beginning of the Atom

5. Alchemists When: 1000’s of years ago Where: Asia and Europe What: Their work developed into what is now modern chemistry. Why: Trying to change ordinary materials into gold and search for everlasting life.

7. Democritus He lived in ancient Greece (460 BC- 370 BC) He said that all matter was made of small indivisible particles Called those particles “atomos” Had NO PROOF! Means INDIVISIBLE

8. Disagreed with Democritus (also in Ancient Greece) All Matter was based upon Nature… Earth, Air, Fire, Water Aristotle (384-322 BC) Matter is continuous – it can be infinitely broken down into smaller & smaller pieces (you can cut it in half forever) Everybody believed him because he was more popular than Democritus Z:\video streaming\Some_Ancient_Thoughts_About_Matter__Democritu s_and_Aristotle.asf

9. John Dalton (1766-1804) Democritus’ idea of the atom was largely ignored until an English schoolteacher did some experiments over 2000 years later, he was… He basically had the same idea as Democritus, but he had PROOF!

10. Dalton’s Atomic Theory 1) All matter is made up of indivisible atoms. 2) All atoms of one element are exactly alike, and they are different from atoms of other elements 3) Compounds are formed by a combination of 2 or more different kinds of atoms (in simple whole number ratios). 4) A Chemical Reactions is a rearrangement of atoms. Video

11. Dalton’s Model of the Atom He believed the atom was a solid sphere An analogy to his atom would be…

12. Thomson’s Cathode Ray Tube Experiments JJ Thomson (1856-1940) used the cathode ray tube to “look” inside the atom Click here to listen to him talk about it

13. Z:\video streaming\Identifying_the_Electron.wmv

15. But, what was that green light? Electricity was passed from one electrode to the other The light always started at the negative end and moved towards the positive end The light ALSO always moved towards a positive magnet and away from a negative magnet http://www.youtube.com/watch?v=7YHwMWcxeX8&fe ature=related http://www.youtube.com/watch?v=7YHwMWcxeX8&fe ature=related

16. Z:\video streaming\Discovering_Electron_Charge.wmv

17. Thomson’s Big Conclusion… Therefore, because opposites attract, he concluded that the cathode ray must be composed of very tiny negatively charged particles. Because the cathode ray was coming out of the negative end (cathode) of the tube and going toward the positive end (anode) He called these particles… Negative end Positive end -------

18. ELECTRONS!! (Discovered by J.J. Thomson!) The discovery would change Dalton’s model of the atom because now there is something inside it But, because the overall charge in an atom is neutral, there be something giving it a positive charge to balance out the electrons (negative) Let’s look at Thomson’s model of the atom…

20. Thomson’s Plum Pudding Model of the Atom He believed the atom was made of positively charged stuff with negatively charged particles scattered throughout Why the “plum pudding” model? What is “plum pudding”?

21. An American analogy to his atom would be… Plum Pudding is an English dish sort of like bread pudding with raisins in it. …Is like… But we’ll still refer to it as the “Plum Pudding” model in class

22. Rutherford and his gold Discover the Nucleus!!

23. Ernest Rutherford In 1908, Rutherford performed the Gold Foil Experiment. AND won a Nobel Prize! In it, he shot alpha particles (very small, very dense, very fast particles) at a thin layer of gold foil. (1871-1937) Smartest person in New Zealand – so they gave him all of their money to study in England.

24. Z:\video streaming\Identifying_the_Nucleus.wmv

25. Alpha particles source Gold foil Detector screen Alpha particles Thomson’s Atom Alpha particles He expected all of the alpha particles to go straight through It would be like if you were shooting bullets at a cake…all of the bullets (or alpha particles) would go straight through the cake (or gold foil atoms)

26. But, what he found was this… http://www.mhhe.com/physsci/chemistry/es sentialchemistry/flash/ruther14.swf

27. Did you notice how most of the alpha particles went straight through but (unexpectedly) a very tiny amount were deflected at odd angles? That could only happen if there was something very tiny in the atom that was dense enough to deflect the alpha particles. Like this…

28. That thing was….. THE NUCLEUS!! Rutherford discovered the Nucleus!!

29. To recap the Gold Foil Experiment

30. Rutherford’s Nuclear Model of the Atom Key Points… 1) The atom is mostly empty space. 2) The center of the atom is small, dense, heavy and positively charged. It’s called the Nucleus 3) Electrons are moving around the nucleus, not in the nucleus Nucleus Electrons

31. Another analogy… If the atom were Lincoln Financial Field, the nucleus would be a pea on the fifty-yard line The atom is mostly empty space!

32. A Rutherford Atom Analogy In which the peach pit is the nucleus and the rest of the peach represents the electrons buzzing around

33. After Rutherford’s model came several other scientists … Together they discovered the “other” parts of the atom: Protons (+ charge) and Neutrons (no charge).

34. ATOM: The smallest unit of matter that maintains it’s properties. Made of Protons (+), Electrons (-), and Neutrons (0). Mostly empty space Small dense nucleus (has protons and neutrons) LARGE (mostly empty) electron cloud (has electrons)

35. The Modern Atomic Theory 1) All matter is composed of tiny particles called atoms. 2) Atoms can be subdivided into subatomic particles [Protons, Neutrons, & Electrons] 3) Atoms of a given element can have different masses called Isotopes 4) Atoms combine in simple whole number ratios to form compounds 5) Atoms can be separated and rearranged during chemical reactions. 6) Law of Conservation of mass holds true

36. Diagram http://www.youtube.com/watch?v=QATa1ev uSec&feature=related

37. On your paper, draw another picture labeling the Parts of an Atom

38. Atom (neutral) Nucleus (positive charge) Electrons (negative charge) Protons (positive charge) Neutrons (positive charge)

39. Conclusion Z:\video streaming\Matter__Building_Blo cks_of_the_Universe__Review _.asf The Atom The Universe

40. How Atoms Differ What we have learned, is different atoms = different Elements So… the Periodic Table is a whole list of atoms Atomic Number - # of Protons in the atom, Identifies the atom No two atoms have the same atomic number, it’s like a “Fingerprint” Atoms have no charge (they’re neutral), because… #protons (+ charge) = #electrons (- charge) Hydrogen 1 H 1.0079 Element Name Element Symbol Atomic Number Atomic Mass

41. ElementAtomic Number ProtonsElectrons Pb82 8 30 82 88O Zn30 What about the Neutrons? Mass Number: Rounded Version of atomic mass = # Protons + # Neutrons

42. WS “Subatomic Particles” 1.Lithium (Li) Atomic Number _________ Atomic Mass _________ # Protons _________ # Neutrons_________ # Electrons_________ 3 7 3 4 3

43. Reading symbols X Mass number Atomic number A Z Atomic Symbol AXZ Formulas  Contains:  symbol of the element  mass number  atomic number

44. Try it… Find the… Atomic number Mass number # of protons # of neutrons # of electrons = 3 = 7 = 3 = 4 = 3

45. How about this one… Find the… Atomic number Mass number # of protons (p + ) # of neutrons (n 0 ) # of electrons (e - ) Ca 41 20 = 20 = 41 = 20 = 21 = 20

46. Two atoms of the same element with a different mass number. Same number of protons Different number of neutrons They relate to the elements b/c they are different forms of the same element

47. How are Isotopes Written? 2 Different Ways 1) Element–Mass number Ex: Na-23 or Na-22 OR 2) AXZ Format: 2322 Na 1111

48. Example Same Protons b/c same element Different Mass Number b/c different number of neutrons Z:\video streaming\Isoto pes.asf

49. Isotope AXZ Format ATOMIC #MASS ##PROTONS #ELECTRONS #NEUTRONS Neon-22 Neon-46 Neon-17

50. Average Atomic Mass aka- Atomic Mass An AVERAGE of the masses of all the isotopes of an element. Mass is measured in amu Atomic Mass Units: Unit of measurement for atoms and their parts Z:\video streaming\Average_and_Atomic_Mass_of_Isotopes.wmv

51. Why does atomic mass have a decimal? Because… It is an average of all of the naturally occurring isotopes of that atom Different elements have several isotopes with different masses that occur naturally here on Earth. Hydrogen has 3 naturally occurring isotopes! http://www.youtube.com/watch?v=Jdtt3LsodAQ&feature=related H - 1H - 2H - 3 This might be confusing……since you can’t have a fraction of a proton or neutron!! (like you can’t actually have 2.5 children)