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Section 2.2

This article explores the discovery of atomic structure through the study of cathode rays and radioactivity. It covers the findings of J.J. Thompson and Ernest Rutherford, as well as the discovery of electrons, protons, and neutrons.

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Section 2.2

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  1. Section 2.2 Discovery of Atomic Structure

  2. Cathode Ray • Mid 1800’s: scientists studying electrical discharge through tubes pumped almost empty of air. • High voltage produces radiation in tube called cathode rays (originates from negative electrode- cathode) • Cause materials (like glass) to fluoresce (give off light) • Used in TVs • Stream of particles or new form of radiation?

  3. The Electron • Streams of negatively charged particles were found to emanate from cathode tubes. • J. J. Thompson is credited with their discovery (1897).

  4. The Electron Thompson measured the charge/mass ratio of the electron to be 1.76  108 coulombs/g.

  5. Millikan Oil Drop Experiment Once the charge/mass ratio of the electron was known, determination of either would yield the other.

  6. Millikan Oil Drop Experiment Millikan determined the charge on the electron in 1909. • 1.60 x 10-19 C • 9.10 x 10-28 g

  7. Radioactivity: • The spontaneous emission of radiation by an atom. • First observed by Henri Becquerel. • Also studied by Marie and Pierre Curie.

  8. Radioactivity • Three types of radiation were discovered by Ernest Rutherford: •  particles •  particles •  rays

  9. How do these particles differ ?

  10. The Atom, circa 1900: • “Plum pudding” model, put forward by Thompson. • Positive sphere of matter with negative electrons imbedded in it.

  11. Discovery of the Nucleus Ernest Rutherford shot  particles at a thin sheet of gold foil and observed the pattern of scatter of the particles.

  12. The Nuclear Atom Since some particles were deflected at large angles, Thompson’s model could not be correct.

  13. The Nuclear Atom • Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom. • Most of the volume of the atom is empty space.

  14. Other Subatomic Particles • Protons were discovered by Rutherford in 1919. • Neutrons were discovered by James Chadwick in 1932.

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