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Nuclear History

Nuclear History. Chemistry 332. 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. 1. Pre-Atomists c. 650 BC.

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Nuclear History

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  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

  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. 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 Atoms or Not?

  7. Who won? • Aristotle’s theories could be easily understood • The four (5) elements could be easily seen and understood • Democritus had no proof Smoke =air fire 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 • CO2- 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 H2O --> 2 H2 + O2

  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 tube • Found that running electricity through a gaseous element produced a stream of particles

  16. Experimented with magnets -

  17. Experiment #2 +

  18. The stream of particles was … Negative Conclusions? • 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 Positive particles Uranium Gold foil (0.00006 cm thick) Detection screen

  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 Positive particles Uranium Gold foil Detection screen

  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

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

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

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

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

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

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

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

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

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