UH PHYS 1302 - Ch31 (20 pages)

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Ch31



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Ch31

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ch 31 study guide


Pages:
20
School:
University of Houston
Course:
Phys 1302 - Introductory to Physics II

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Chapter 31 Atomic Physics 1 Early Models of the Atom Thomsom Model Plum Pludding J J Thomson discovered the electron in 1897 The electron is much less massive than the atom and negatively charged Since atoms are neutral he proposed that atoms have an internal structure that includes negatively charged electrons and some positively charged matter In Thomson s plum pludding model the electrons are embedded in a uniform distribution of positive charge like raisins spread throughout a pudding The Rutherford Model A Miniature Solar System Ernest Rutherford and colleagues Hans Geiger and Ernest Marsden decided to test Thomson s model They shot a beam of positively charged particles at a thin sheet of gold foil The particles they used were alpha particles which we know now are the nuclei of helium atoms Rutherford expected that the alpha particles would be deflected only a little small angles since the alpha particles are heavy compared to the electron and the positive charged is spread out He also expected all the alphas to be deflected in approximately the same way since the positive charge is spread out everywhere The results did not match the expectations Most of the alpha particles passed through the foil without being deflected at all The few alphas that were deflected were deflected at very large angles In fact some of the alphas deflected almost L Whitehead 1 Phys 1302 perfectly backwards like they bounced off a wall Rutherford said It was almost as incredible as if you fired a 15 inch shell at a piece of tissue paper and it came back and hit you Based on these results Rutherford proposed that the atom has a structure similar to the solar system with the negatively charged electrons orbiting a positively charged nucleus that contains most of the atom s mass When the alpha particles bounced back it s because they hit the nucleus directly He calculated that the radius of the nucleus must be smaller than the diameter of the atom by a factor of around 10 000 If the



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