Development of Atomic Theory (and various people along the way)

1. Development of Atomic Theory(and various people along the way)

2. The Greek Philosophers (THE GREEK PHILOSOPHERS) who thought but did not experiment

4. Leucippus Teacher of Democritus First proposed that if matter was divided and divided over and over and over, it would reach a smallest particle he called an ATOM. MATTER IS PARTICULATE!

5. Why don’t you hear about Leucippus? His writings didn’t survive. We know about him from his student Democritus who wrote a lot more about the idea of atoms.

6. Democritus (470 - 380 BC)

7. Proposed that all matter is made up of tiny, invisible particles he called ATOMS. (this was from Leucippus)

8. Also proposed that atoms had differentSHAPESthat gave them different properties.(Democritus’ own ideas)

9. Aristotle ( 384 – 322 BC )

10. Aristotle believed… If you divided matter over and over and over, you would still be able to divide it. Matter iscontinuous.

11. Disagreed with Democritus on the basis that there was nothing to hold atoms together. Also, what’s in between the atoms?

12. Aristotle believed everything was made up of : • Water • Earth • Air • Fire

14. Alchemy • Based on Aristotle’s beliefs that all different substances had different proportions of earth, air, fire, and water, although later other elements were added. • By changing proportions, alchemists could achieve two goals: • Change other metals to gold • Discover substance promoting eternal youth

15. Alchemy Problem with alchemy: IT DOESN’T WORK!!

16. These beliefs lasted hundreds of years –until CHEMISTRYreplaced ALCHEMY

17. WHY NOT CHANGE IDEAS ABOUT MATTER? • The Roman Catholic Church supported Aristotle’s views because he also proposed, along with Ptolemy, the idea of a geocentric universe: everything revolves around the Earth. This fit Catholic theology. • The Catholic church provided most education, so what people learned was Aristotle. (so nothing changed for a long, long time)

18. Why did things change? • Roger Bacon (13th century): first stated scientific method • Copernicus (14th century): proposed heliocentric universe: planets orbited sun. • Martin Luther (15th century): started the Protestant reformation (break with Catholic Church) • Henry VIII (15th century): like Luther (but independently) broke with Catholic Church and started educational system not run by Catholic Church.

19. Antoine Lavoisier(1743 – 1794)

20. Law of Conservationof Mass:During a chemical reaction, the total mass of all reactants = total mass of all products

21. 1754-1836

22. Law of Constant Composition A compound always contains the same elements in the same proportion by mass.

23. Atomic Theory of Matter

24. Elements are made up of small indivisible particles (atoms) • All atoms of the same element have identical properties. • Atoms cannot be created, destroyed or transformed during a chemical reaction. (law of conservation of mass)

25. Compounds are formed when atoms of different elements combine with each other in simple numerical ratios. • In a compound, relative numbers and kinds of atoms are constant. (law of constant composition)

26. Dalton’s Law of Multiple Proportions • When two elements form a series of compounds, the ratios of the masses of the second element combine with a fixed mass of the first element can always be expressed as the ratio of small whole numbers. • When two elements can combine to form more than one compound and the same amount of the first element is used in each, then the ratio of the amounts of the other element will be a whole number ratio.

27. Dalton’s Atoms Solid spheres Different elements have different masses

28. Benjamin Franklin (1706–1790) Concluded from experimentation that there were 2 types of electric charge. +,-

29. Michael Faraday (1791-1867) Suggested that the structure of the atom is related to electricity

30. Cathode Ray Tubes (mid 1800’s) Cathode (-) Anode (+)Electric plates (one +, one -) partially evacuated glass tube that produces a beam between the cathode and anode

31. When filled with a gas that fluoresced in the presence of electric charge, a stream of radiation(the cathode ray) was seen from cathode to anode.

32. J. J. Thomson (1856 – 1940) Investigated cathode rays.

33. Thomson and Cathode Rays • If electrically charged metal plates were placed above and below the ray, the ray bent. It ALWAYS bent toward the positive plate. • If a different metal was used to make the cathode, he got the same results. • If a different gas filled the tube, he got the same results.

34. The Atom had a Substructure • Cathodes made of different metals gave off identical particles. Meaning: particles are in all atoms! • Particles were attracted to positive plate and repelled by negative plate. Meaning: particles have negative charge! • Because original atoms were neutral, same amount of positive charge remained in the atom as left with the electrons.

35. What are the particles? electrons

37. THE OIL DROP EXPERIMENT Robert Millikan (1868 – 1953) Calculated the mass of an electron (9.11 x 10-28 g) and its charge (-1 fundamental charge units)

38. Oil Drop Experiment

39. Henri Becquerel(1852 – 1908) • Discovered radioactivity in 1896

40. Marie Curie (1867-1934) • Discovered that radiation accompanies changes in identity of the radioactive atom

41. Others also investigating radioactivity:Pierre CurieOtto HahnLise Meitner

42. Ernest Rutherford (1871 – 1938) Discovered: Alpha particles and Beta particles

43. The Gold Foil Experiment • Alpha particles have a very small size and mass and a positive charge. • Rutherford shot alpha particles at a very thin sheet of gold foil. • Rutherford expected alpha particles to “blast” a hole in the gold foil and go straight through because he thought the positive charge was evenly spread through the foil.

44. Animation of the Gold Foil Experiment

45. What he Saw was NOT what he expected

46. “It was quite the most incredible event that has ever happened to me in my life. It was almost as if you fired a 15-inch shell into a piece of tissue paper and it came back and hit you.”

47. WHAT HE SAW AND WHAT IT MEANT: part 1 Data: Most alpha particles went straight through the foil as if it was not there. Conclusion: there was nothing where the atoms passed through. Meaning: most of gold atom is empty space.

48. WHAT HE SAW AND WHAT IT MEANT: part 2 Data: Some alpha particles bounced straight back from the foil toward the alpha particle source. Conclusion: the alpha particle hit something much more massive and dense than itself. Meaning: There is a very small part of the atom that contains most of the atom’s mass.

49. WHAT HE SAW AND WHAT IT MEANT: part 3 Data: Some alpha particles deflected: bent away without bouncing. Conclusion: the positive alpha particle was repelled by something else that was positive. Meaning: The part of the atom containing the mass has a positive charge.

50. Rutherford’s Theory The atom contains a center which is • Very small • Positively charged • Contains almost all the atom’s mass. He called it the NUCLEUS His model was the NUCLEAR MODEL