2-Discovery of Nucleus - 5.111 Lecture Summary #2 September...

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5.111 Lecture Summary #2 September 11, 2009 RaBr 2 α particles detector RaBr 2 α particles detector Au foil (~10 -9 m thick) Reading for today: A.2-A.3 (pgs F10-F13), B.1-B.2 (pgs F15-F18), & Section 1.1 Reading for Lecture 3: Sections 1.2 & 1.4 DISCOVERY OF NUCLEUS - 1911 - E. RUTHERFORD Thomson proposed a “plum pudding” model of the atom: electrons embedded in a cloud of positive charge. Ernest Rutherford interested in studying α emission from newly discovered radioactive elements α particles are He 2+ - this was unknown in 1911 A detector was built that could swing around to the front side Positive pudding Negative electron plums Count rate = 132,000 α particles / min Count rate = 132,000 α particles / min !!!
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5.111 Lecture Summary #2 September 11, 2009 RaBr 2 α particles detector Au foil (~10 -9 m thick) detector for backscattering Backscattering was detected!! Probability (P) of backscattering: P = count rate backscattered = 20/132, 000 = 2 x10 -4 count rate of incident particles Interpretation: Plum pudding model disproved! New atomic model Since the vast majority of α particles pass through the Au foil undeflected, the Au atoms are mostly empty .
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This note was uploaded on 02/08/2010 for the course CHEM 143 taught by Professor Mike during the Spring '10 term at École Normale Supérieure.

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2-Discovery of Nucleus - 5.111 Lecture Summary #2 September...

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