Ch400Ch2LN2 - Chem 400 Chapter 2 Lecture Notes Part 2 Going...

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Chem 400 Chapter 2 Lecture Notes Part 2 Going Further: The Structure of Atoms Dalton thought that atoms were the smallest particle of matter, but through a series of experiments starting in the late 1800’s, this was proved to be incorrect. Cathode ray tubes had been developed and were being studied: Cathode rays were discovered to have a _______ charge by the following experiment: In 1897, JJ Thomson proposed that cathode “rays” were in fact particles with a negative charge, electrons ! Thomson also discovered that it didn’t matter what the cathode material was composed of, identical electrons were given off. Thomson also conducted a series of experiments where he varied the strength of the electric and magnetic fields and carefully measured the amount of deflection of the electron beam. Thomson was able to determine the _______________ratio of an electron, 1.758 C/g. However, scientists wanted to know the actual charge and mass, not just the ratio. In 1909, Robert Millikan conducted his famous Oil-Drop Experiment : From this experiment, Millikan obtained the actual charge on an electron, 1.60x10 -19 C. And from this charge and Thomson’s charge/mass ratio, the exact mass of an electron was calculated to be 9.10x10 -28 g. So from these experiments, scientists deduced that atoms were made up of even smaller subatomic particles, one of which was the electron.
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Since electrons have a negative charge, while atoms are neutral, scientists also realized that there had to be at least 1 more subatomic particle with a positive charge. But where was this particle in the atom and what did it look like? A popular model of the atom at this time was
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Ch400Ch2LN2 - Chem 400 Chapter 2 Lecture Notes Part 2 Going...

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