02a-Atoms_Molecules

02a-Atoms_Molecules - Chapter 2 Atoms, Molecules and Ions...

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1 Chapter 2 Atoms, Molecules and Ions Dalton’s atomic theory of matter (1808) (based on macroscopic chemical observations) 1. Elements are composed of extremely small particle building- blocks called atoms 2. All atoms of a given element are identical and have identical properties that differ from those of other elements 3. Atoms cannot be created, destroyed, or transformed into atoms of another element by chemical reactions ( law of conservation of mass ) 4. Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms
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2 Discovery of electrons (Thomson, 1897) Cathode rays generated by different materials are made up of negatively charged particles ( electrons ) Electrons are deflected by electric and magnetic fields –> electron charge-to-mass ratio
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3 Measuring the electron charge: Millikan’s oil drop experiment e e e e e e Oil drops charged with 0,1,2,3,… extra electrons. Measuring the electron charge: Millikan’s oil drop experiment Oil drops with 0,1,2,3,… electrons. The electric field pulls the drops upward with a force proportional to the number of electrons. Adjusting the electric field until a drops was stationary Millikan e All oil drops tend to fall because of gravity. drops was stationary, Millikan could infer the charge on drop. e e e e e Charge always a multiple of 1.602 X 10 19 Coulombs, the electron charge.
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4 Electron mass m e ~ 9.1 x 10 -28 g * ~2000 times smaller than mass of hydrogen atom Thomson’s atomic model, with electrons embedded in a matrix of positive charge Quickly eliminated by experiment Rutherford’s experiment crucial result An α particle is a cluster of 2 protons and 2 neutrons. (radium)
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5 Explanation of Rutherford’s experiment Existence of a small Existence of a small dense nucleus is the explanation why only a small fraction of the a particles are deflected.
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This note was uploaded on 01/17/2010 for the course CHEM 121 taught by Professor Wyzlouzil during the Spring '07 term at Ohio State.

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02a-Atoms_Molecules - Chapter 2 Atoms, Molecules and Ions...

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