1. Nuclear History Chemistry 332

2. Seven Important Eras  Pre-atomists  Early atomists  Late atomists  Plum pudding model  Nuclear model  Planetary model  Quantum mechanical model  For a more complete listing check out the nuclear history page For a more complete listing check out the nuclear history page

3. 1. Pre-Atomists c. 650 BC  What was the world made of?  Thales- water because it was everywhere  Anaximenes- air because it was around and over us  Heraclitus- fire because it was an agent of change  Empedocles- offered ideas of opposite pairs of earth/air and fire/water  Aristotle- combined all above ideas and added the 5th element “ether”

4. 2. Early Atomists c. 470 BC- 55BC  Leucippus- if you cut something in half enough times, you will reach a particle that can no longer be cut

5. Democritus  Indivisible = atomos  atoms were distinct in size and shape  each element was therefore unique  substances of the world were made from combinations of different atoms  substances could be changed by altering the mixture of the composition

6. Atoms or Not?  Aristotle’s followers said how can you have a piece of finite matter so small it can’t be cut in half.  Atoms don’t make sense  Besides- look at the “proof” of the 5 element theory  Democritus’ followers said that you just can’t keep cutting something in half.  There must be some endpoint to the mess somewhere that is the basic building block of the universe

7. Who won?  Aristotle’s theories could be easily understood  The four (5) elements could be easily seen and understood  Democritus had no proof fire Smoke =air ashes = earth

8. A little theory goes a long way  Because of this, the ideas of Aristotle prevailed “scientific” thinking for a long time  How long?  The next major theory doesn’t arrive until the 1800’s!  Aristotle’s ideas lasted about 2000 years!!!

9. Were they Scientists?  Were Aristotle and Democritus scientists?  What makes a scientist?  Performing controlled experiments  Since neither could really did any experiments they were not scientists

10. So what were they?  What do you call a theorists who does not perform experiments?  A Philosopher um…atoms

11. 3. Late Atomists  John Dalton (1766-1844) reinvents the ideas of atoms  Dalton begins to perform experiments with matter which chips away at Aristotle’s theories  Multiple proportions- atoms can combine in different whole number ratios to make different substances:  CO -carbon monoxide is very different from  CO 2 - carbon dioxide  Dalton found that different masses of different elements combined in simple whole number ratios of masses

12. So what?  Two of Dalton’s contemporaries were Nicholson and Carlisle  If Aristotle was right, then none of the four elements (earth, air, fire, or water) could be broken down into further elements  Right about this time, two Italians, Volta and Galvani each invented an electrical stack which could deliver a constant electrical current  Nicholson and Carlisle ran electricity through water and….  It decomposed!!!

13. What did it decompose into?  They thought:  HO --> O + H  But it came out in a 2:1 ratio of H:O  Therefore (after others’ experiments):  2 H 2 O --> 2 H 2 + O 2

14. And So?  Atoms came to be accepted in the general world  So what was the next question?  What’s inside atoms?

15. 4. Plum Pudding Model  JJ Thomson (1856-1940)  Was using a Crooke’s tubeCrooke’s tube  Found that running electricity through a gaseous element produced a stream of particles

16. Experimented with magnets -

17. Experiment #2 +

18. Conclusions?  The stream of particles was …  Negative  Overall, atoms are...  Neutral  Therefore, atoms must also contain charges that are...  Positive

19. Plum Pudding Model  Thomson figured out that atoms contained positive and negative charges but had no idea how they were arranged.  He guessed there was a diffuse positive cloud with negative charges randomly distributed Diffuse positive cloud - - - - - -

20. 5. Nuclear Model  Ernest Rutherford (1910)  Performed the famous gold foil experiment

21. Set-up Detection screen Gold foil (0.00006 cm thick) Uranium Positive particles

22. Hypotheses  If Thomson was correct and atoms contained only diffuse positive charge then…  Most of these heavy positive particles should go right through, right?

23. Experiment Detection screen Gold foil Uranium Positive particles

24. What’s going on?  Most are going straight through  But some are being deflected  How much can they be deflected?

25. Continuation

26. HOLY ATOMS!  What conclusions can be drawn here?  There is something inside an atom that is heavy enough to deflect a particle straight back  That “something” must be positively charged  It must also be very small

27. Conclusions?  Heavy, dense, positively charged, small piece of the atom is….  The nucleus!!  Lets watch this animated Lets watch this animated

28. Nuclear Theory  Nucleus of atom contains @99.9% of the mass of the atom  Nucleus is positively charged  Electrons orbit the nucleus randomly and take up @99.9% of the space  Electrons are negatively charged

29. Side note  Notice that Rutherford has no mention of the neutron  Neutron wasn’t discovered until 1933 by Chadwick

30. 6. Planetary Model  Bohr proved that electrons don’t orbit randomly around the nucleus  He postulated that they orbit the nucleus like planets do around the sun  They circle in well-defined energy levels  Jumping to and from energy levels absorbs or releases energy

31. Atomic Line Spectra Gas such as Neon or Hydrogen Prism or diffraction grating Expected to see a nice progression of color just like normal BUT…..

32. Atomic Line Spectra Gas such as Neon or Hydrogen Prism or diffraction grating Instead he got just a few discrete lines of color!

33. WHAT GIVES?  Why are only a few lines of color seen instead of the whole spectrum?  Bohr postulated that electrons exist at specific energy levels or quanta.  Electrons can only jump from specific energy level to specific energy level and not anywhere in between.

34. Planetary Model Nucleus e-e- e-e- e-e- e-e-

35. For Example: Energy level 1 Energy level 2 OK NO!

36. For Example: Energy level 1 Energy level 2 To go from 1 to 2 energy must be absorbed or put into the system of a specific amount…

37. Or… Energy level 1 Energy level 2 When you drop from a higher energy level to a lower one energy is released in a specific amount…

38. For Electrons Nucleus 3  2 2  1 3  1

39. 7. Quantum Mechanical Model  Heisenberg, Planck, Einstein, etc  1900’s to today  The world of the atom is so small that just by us looking for small pieces changes where they are  Thus we can’t be sure exactly about these energy levels so….

40. Electron Cloud Model Nucleus e-e- e-e- e-e- e-e-