Chapter7and8Lecture

Chapter7and8Lecture - Chapter 7: Electrons in Atoms A...

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6 Copyright: 2010 Prof. Magde Chapter 7: Electrons in Atoms • A century ago, soon after the electron was discovered by Thompson, there were a number of ideas for how electrons might exist in atoms. • One idea had a big sphere with negative and positive charge distributed throughout. Perhaps small negative electrons in a positive background, like raisins in pudding. Or chocolate chip cookies. Rutherford ended that idea when he discovered that the nucleus was a very tiny, hard, positively charged piece in the middle. • Then there was Bohr’s idea that tiny electrons might fly around the little (but heavy) nucleus like planets around the sun. • You still see some high school books show circular orbits with 2 electrons in the first ring, and eight electrons in the second ring. But this is just wrong.
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6 Copyright: 2010 Prof. Magde Chapter 7: Electrons in Atoms • The old quantum theory was that model due to the Danish physicist Bohr. He imagined the electron to move around the nucleus like a planet around the sun. That orbit can be calculated using high school physics, or at least AP physics or college non-calculus physics. Bohr added one piece: that the electron had an integral number of wavelengths that fit into the orbit around the nucleus. Hence only certain orbits were allowed. This gives the formula we had earlier for the possible energies of electrons in H, both the dependence on 1/n 2 and even the numerical coefficient in joules. • You still see this in some high school chemistry texts. You never see it in a college text, because that model does not work, even after being extended somewhat. • Our current model takes the electron to be a large cloud, large relative to a nucleus.
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6 Copyright: 2010 Prof. Magde Chapter 7: Electrons in Atoms • This modern cloud model does not use equations from high school physics, but it does use another equation. H = E • This is named Schroedinger’s Equation. We cannot take time to learn how it really works, but here is the basic idea: is a “wavefunction” -- something that tells us where the electron cloud is and what it is doing. That is, its position and momentum. E is the total energy associated with a particular . H is the key. It has two parts: one tells us how to find the kinetic energy from , the other tells us how to find the potential energy. So this equation is saying something like the total energy is the sum of the potential energy plus the kinetic energy.
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6 Copyright: 2010 Prof. Magde Chapter 7: Electrons in Atoms • The fact that is rather cloud-like is related to the Heisenberg uncertainty principle. You can only learn so much about position and motion. This means that the electron cannot collapse
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Chapter7and8Lecture - Chapter 7: Electrons in Atoms A...

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