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History of the Atom

History of the Atom. History of the Atom The Greeks – 200 BC Dalton – 1800s Thompson – 1900s Rutherford – 1910 - Rutherford Marble Activity!! Chadwick - 1932 Bohr’s Model – 1922 Quantum Mechanical Model – 1926. ~. ~. Fire Water Earth Air. The Greeks History of the Atom.

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History of the Atom

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  1. History of the Atom • History of the Atom • The Greeks – 200 BC • Dalton – 1800s • Thompson – 1900s • Rutherford – 1910 - Rutherford Marble Activity!! • Chadwick - 1932 • Bohr’s Model – 1922 • Quantum Mechanical Model– 1926

  2. ~ ~ Fire Water Earth Air The GreeksHistory of the Atom • Not the history of atom, but the idea of the atom • In 400 B.C the Greeks tried to understand matter (chemicals) and broke them down into earth, wind, fire, and air. • Democritus Greek philosopher

  3. Greek Model • Democritus was a Greek philosopher who first proposed the idea of a small indestructible particle that composes all matter • Democritus called this building block the ATOM • No experiments to support idea “To understand the very large, we must understand the very small.” Time Period: 200 B.C Democritus’s model of atom No protons, electrons, or neutrons Solid and INDESTRUCTABLE

  4. The Greek Philosophers Democritus Aristotle Did not believe in particles that could not be seen. It could be subdivided indefinitely There were no “particles” that matter was composed of Continuous theory of matter – matter is made up of blobs • Proposed the word atom(from Greek word atomos), meaning indivisible, because he believed that all matter consisted of such tiny units with voids between. • Matter must be small particles which were indivisible • Discontinuous theory of matter – matter is made up of particles

  5. Who Was Right? • Greek society was slave based • Did not experiment • Greeks settled disagreements by argument • Aristotle was the more famous philosopher- He won! • His ideas carried through middle ages. California WEB

  6. Dalton Model of the AtomTime period: 1800’s John Dalton- England Teacher- summarized results of his experiments and those of other’s. The first individual to propose an atomic theory based on experimentation. Combined ideas of elements with that of atoms

  7. John DaltonTime period: 1800’s • All matter consists of tiny particles,. Dalton, like the Greeks, called these particles atoms. Believed that they cannot be subdivided, created, or destroyed • BUT we now know that atoms can be broken apart or merged together during nuclear reactions 2. Atoms of one element can neither be created nor changed into atoms of any other element. • They obey the law of conservation of mass

  8. John Dalton • All atoms of the same element are the same in mass, size and other properties. • BUT we now know that not all atoms of the same element have identical properties 4. Atoms of one element differ in size, mass, and properties from atoms of other elements.

  9. John Dalton 5. In compounds, atoms of different elements combine in simple, whole number ratios.

  10. History of the Atom (1:50min)

  11. Thomson Model of the Atom J. J. Thomson - English physicist. 1897 Made a piece of equipment called a cathode ray tube. It is a vacuum tube - all the air has been pumped out.

  12. ThomsonTime Period: 1900’s Description of Experiment • Cathode Ray Tube • Rays appear on a screen • Thomson looked to see how a magnet affected the rays

  13. Thompson Video

  14. Source of Electrical Potential Stream of negative particles (electrons) Metal Plate Gas-filled glass tube Metal plate A Cathode Ray Tube Zumdahl, Zumdahl, DeCoste, World of Chemistry2002, page 58

  15. Thomson’s Experiment voltage source ON - OFF + Passing an electric current makes a beam appear to move from the negative to the positive end

  16. + - Thomson’s Experiment voltage source ON - OFF + By adding an electric field… he found that the moving pieces were negative.

  17. ThomsonTime Period: 1900’s • Cathode Ray Tube • What happened? • Negative side of the magnet repelled • Positive side of magnet attracted rays

  18. ThomsonTime Period: 1900’s • Conclusions from the Cathode Ray Tube (CRT) • Atom has a negative particle called the ELECTRON • Analogy for model: Chocolate Chip Cookie (Plum Pudding)

  19. Thomson’s Conclusions • He concluded that atoms do contain subatomic particles - atoms are divisible into smaller particles. • This conclusion contradicted Dalton’s postulate (of the atom being indivisible), and… • Therefore was not widely accepted by fellow physicists and chemists of Thomson’s day.

  20. Thomson’s Plum-Pudding Model Positively charged ball of matter with electrons embedded in it. Zumdahl, Zumdahl, DeCoste, World of Chemistry2002, page 56

  21. J.J. Thomson

  22. Ernest Rutherford Time Period: 1910 • Learned physics in J.J. Thomson’s lab. • Used the “Gold-foil Experiment” to disprove his mentor’s Plum Pudding model

  23. Gold Foil Experiment • Video (55sec)

  24. What He Expected • The alpha particles to pass through without changing direction (very much) • The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles California WEB

  25. What he expected… California WEB

  26. Because he thought the mass was evenly distributed in the atom. - - - - -

  27. - - - - - - - - - - - - - - - - - - - - - - - - - Because, he thought the mass was evenly distributed in the atom

  28. What he got…

  29. Gold foil experiment video 2 (1:30min)

  30. Ernest Rutherford Time Period: 1910 • What happened? • Most alpha particles went through • Some deflected at different angles • Some bounced right back

  31. Rutherford Marble Activity • Read through Introduction and Background Need: • 1 board • paper • 2-3 marbles • Pen/pencil • Lab Notebook

  32. Day 3 • Element Quiz #4 • Finish Rutherford Marble Activity Discussion questions!

  33. Day 4 • **Need periodic tables from now on!! • Finish notes on Rutherford and Chadwick • Structure of Atom • Mass #, Atomic #, Isotopes • Pg2 chart (1/2) • Nuclide Symbols

  34. Density and the Atom • Since most of the particles went through, the atom was mostly empty. • Because the alpha rays were deflected so much, the positive pieces it was striking were heavy. • Small volume and big mass = big density • This small dense positive area is the nucleus California WEB

  35. . . . . . . . . . . . . . . gold foil Interpreting the Observed Deflections . beam of alpha particles undeflected particles . . deflected particle Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

  36. Ernest Rutherford Time Period: 1910 • Conclusions • Atom is mostly empty space • Mass is concentrated in the center (NUCLEUS) • Nucleus is positive

  37. Ernest Rutherford Time Period: 1910 • Analogy for Model: PEACH • Picture:

  38. The Rutherford Atom n + Zumdahl, Zumdahl, DeCoste, World of Chemistry2002, page 57

  39. James Chadwick Time Period: 1932 • Discovered the neutral subatomic particle in the nucleus: NEUTRON

  40. Review • Who discovered the electron? How? • Thomson – cathode ray tube • Negative side of magnet repelled rays • Positive side of magnet attracted rays • Who discovered the proton/nucleus? How? • Rutherford – gold foil experiment • Most particles went straight through gold foil, while some particles were deflected • Showed there was a small dense ball in the center of the atom • Who discovered the neutron? • James Chadwick

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