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Topic: QM, atoms, molecules, geometry  (Read 2698 times)

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Offline Lara

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QM, atoms, molecules, geometry
« on: September 16, 2011, 05:52:14 AM »

In chemistry, you can draw pictures where atoms sit in certain places. These molecular geometries are given in coordinates.

How can this be understood? Quantum mechanics says that the place of quantum state can not be sharp. These may mean expectation values ​​of the places where the chemist is painting the picture? And what is the expected value of an atom?

Can someone tell me something about the quantum mechanical reasoning behind it?

One idea was, that atoms are big enough to neglect quantum effects but than there is the following Problem:

In C60 buckyballs one still can see QM effects. Where is the limit when it is legitimate to ignore the position uncertainty.

So when I see a chemical graph, then where such Carbon atoms are drawn with coordinates (ie at pretty much everything there is in the OC), but then I have QM effects, since I'm still at C60.

Please help.

Thanks and sorry for bad english

Offline Borek

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Re: QM, atoms, molecules, geometry
« Reply #1 on: September 16, 2011, 06:42:39 AM »
Treat these positions as averages. Atoms don't stay precisely there - they are almost always vibrating, rotating and so on. But they do move around these positions. What is important is that even when they move, their general relative positions are always similar (technically we could say topology of the molecule doesn't change).
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