2 - MIT OpenCourseWare http/ocw.mit.edu 5.112 Principles of...

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MIT OpenCourseWare http://ocw.mit.edu 5.112 Principles of Chemical Science, Fall 2005 Please use the following citation format: Sylvia Ceyer and Christopher Cummins, 5.112 Principles of Chemical Science, Fall 2005 (Massachusetts Institute of Technology: MIT OpenCourseWare). http://ocw.mit.edu (accessed MM DD, YYYY). License: Creative Commons Attribution-Noncommercial-Share Alike. Note: Please use the actual date you accessed this material in your citation. For more information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms
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MIT OpenCourseWare http://ocw.mit.edu 5.112 Principles of Chemical Science, Fall 2005 Transcript – Lecture 2 Great. Well, let's get going. Last time we ended up by discovering the electron. We discovered the fact that the atom was not the most basic constituent of matter. But in 1911 there was another discovery concerning the atom, and this is by Ernest Rutherford in England. And what Rutherford was interested in doing was studying the emission from the newly discovered radioactive elements such as radium. And so he borrowed, or he got, from Marie Curie, some radium bromide. And radium bromide was known to emit something called alpha particles. And they didn't really know what these alpha particles were. Now, they did know that the alpha particles were heavy, they were charged and that they were pretty energetic. That is what was known. Of course, today we know these alpha particles to be nothing other than helium with two electrons removed from the helium, helium double plus. Rutherford is in the lab and has this radium bromide, alpha particles being emitted and has some kind of detector out here to detect those alpha particles. And he measures a rate at which the alpha particles touch his detector. And it is about 132,000 alpha particles per minute. That's nice. Then what he does is takes a piece of gold foil and puts it in between the radium bromide emitter and the detector. And that gold foil is actually really very thin. It is two times 10 to the minus 5 inches. Two orders of magnitude thinner than the diameter of your hair. I often wonder how he handled that, but he did it. He put it in the middle here and then went to count the count rate as a result of putting this foil there, and the count rate is 132,000 alpha particles per minute. It didn't seem like that gold foil did anything.
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The alpha particles were just going right through to the detector. It didn't even seem to matter that there was that gold foil. The post-doc that was working on it, Geiger, of the Geiger Counter, was actually disappointed. Gee, that is a boring experiment. But Geiger was even a little bit more unhappy because he had this undergraduate hanging around the lab, this undergraduate named Marsden. And Marsden was really enthusiastic about doing something in the lab. He really wanted to do something. And Geiger, you know, what am I
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This note was uploaded on 10/30/2008 for the course CHEM 5.112 taught by Professor Cummins during the Spring '08 term at Academy of Art University.

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2 - MIT OpenCourseWare http/ocw.mit.edu 5.112 Principles of...

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