beyond thomson atom

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What does Rutherford's experiment teach us about the atom?

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Beyond Thomson’s Atom The discovery of the modern atom

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Thomson’s Model The atom as “raisin pudding” Electrons imbedded in sphere of + electricity

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Implications of the Model Electrons imbedded in sphere of + electricity are very light Nothing in the sphere of positive electricity is very “dense”

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Testing the Model So if we shoot a positively charged projectile at this atom, it will either deflect slightly or go straight through it; maybe some small deflection

Thomson's Prediction:

Thomson's Prediction

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The Rutherford Experiment Ernest Rutherford, U. Manchester 1911 α particles are positively charged ions that are ejected from radioactive elements like Radium α

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The Rutherford Experiment Source ejects alpha (+) particles These particles are collimated into a narrow beam which is “shot” at a very thin film of gold, a dense element The Rutherford Experiment

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The Rutherford Experiment Alpha (+) particles fall on the gold Thomson’s prediction is that if any of the alpha particles get through, they would go straight to the target The Rutherford Experiment

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Rutherford’s Results Rutherford found that most particles did go straight through Indeed, most of the + particles were undeflected; scintillations were largely “dead on” the target

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Rutherford’s Results But. . . some of the particles were deflected Yet a significant number of flashes were seen above and below the center line, and some even when the target was moved behind the foil! Rutherford’s Results

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Rutherford’s Conclusion “It’s as if you had fired a 15-inch shell at a piece of tissue paper, and it came back and hit you” Atom’s Mass is not Evenly Distributed It is concentrated, positively charged He called this concentration the “nucleus” after the nucleus of the cell It is less than 0.1% of the volume of the atom

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Rutherford’s Atom Positive nucleus (with neutrons) and electrons spinning around it

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Rutherford’s Problem He didn’t know how the electrons could orbit the nucleus Accelerating charged particles lose energy Electrons constantly accelerate toward the nucleus So they would spiral into the nucleus Rutherford’s atom is not stable according to classical physics

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Rutherford’s Legacy Rutherford Advanced our understanding of the atom The nucleus is the locus of the atom’s mass and positive charge The electrons are the “engines” for all chemical reactions He gave us, for good or ill, the nuclear “age”

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Chadwick’s Contribution Consider oxygen: atomic mass 16 amu But its atomic number (protons) = 8 amu What makes up the difference? Difference in mass but not in atomic charge NEUTRON 1 amu but 0 charge

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Isotopes The neutrons help hold the nucleus together All atoms with atomic number > 1 have them Two atoms of the same element can differ in the number of neutrons Carbon has 3 isotopes with same chemistry C-12, C-13, C-14

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Isotopes Isotopes of the same element have the same chemistry—same reactions, same properties But isotopes of the same element have different # of neutrons So isotopes of the same element have different masses Combination of all isotope masses What we read on the periodic table More on that next time

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