chem161sg_chapter7

chem161sg_chapter7 - CHAPTER 7 Atomic Structure The...

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CHAPTER 7 Atomic Structure The Subatomic Particles Electron: Experiments done by J.J. Thomson on cathode rays (rays emitted from metal surface when a high voltage difference is applied between two electrodes) revealed the existence of the electrons (electrically charged particles that carry electric current). Thomson's Experiment By measuring the amount of deflection produced by electric and magnetic fields, Thomson was able to calculate the ratio of the charge over the mass of the electron. ( Note: text uses mass/charge ratio ) C = coulomb (the SI unit for electric charge) (negative because of the charge of the particle) -1.76 x 10 8 g C e m e = Robert Millikan: (1909) By measuring the charges on oil drops sprayed through an atomizer, he was able to calculate the charge on the electron. charge on the electron: e = -1.60 x 10 -19 C Using the result of Thomson's Experiment Millikan was able to calculate the mass of the electron. mass of the electron: e/m e = -1.76 x 10 8 C/g -1.60 x 10 -19 C /m e = -1.76 x 10 8 C/g m e = = 9.09 x 10 -28 g (very small) The mass of the electron was calculated to be smaller than the mass of the lightest element which was then known (hydrogen). Therefore, the electron should be a subatomic particle. -1.60 x 10 -19 C -1.76 x 10 8 C/g 1
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The Nuclear Atom - Atoms are electrically neutral. - If atoms contain negatively charged particles, then they should also contain positively charged particles to make the atom neutral. - Question - where is the positive charge located in the atom? Rutherford’s Gold Foil Experiment α (alpha) particles were known from radioactive elements to be helium atoms stripped of their electrons, i.e. He 2+ The experiment was to bombard a gold foil by α -particles and measure where these particles hit the screen behind the gold foil. Results: most of the α -particles went straight through the foil, but some deflected at very large angles. Conclusion: most of the atom is an empty space with a positive center. The mass of the atom is concentrated in the center which is called the nucleus of the atom. The positive mass was called protons (meaning first) by Mosely. The number of protons could be estimated by the deflections of the α -particles in Rutherford's Experiment. The total mass of the atom calculated from the masses of protons was less than the actual mass of the atom. the existence of an equally massive neutral specie in the nucleus was suggested, and in 1932 Chadwick discoverd the third subatomic paticle, the neutron . Name Symbol Mass (g) Mass (u)* Relative Charge proton p 1.6725 x 10 -24 1.0073 +1 neutron n 1.6748 x 10 -24 1.0087 0 electron e - 9.109 x 10 -28 0.0005486 -1 * u = atomic mass unit 2
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Atomic Structure Revisited Review: Rutherford’s model of the atom consisted of a nucleus – positively charged, which is surrounded by electrons. The laws of classical mechanics (Newton’s Laws) used to precisely
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This note was uploaded on 09/30/2011 for the course CHEM 161 taught by Professor Vacillian during the Spring '08 term at Rutgers.

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chem161sg_chapter7 - CHAPTER 7 Atomic Structure The...

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