How do chemists know what the structure of a molucule is?

Question

I got hired to teach high school physics but I picked up a chemistry class. I can handle most of it but I'm rusty. I was showing the structure of some molecules like benzene ring, water, etc. One of my students asked me how do we know what the shape of these things are if they are so small and I honestly couldn't answer. Can someone explain to me (so I can explain it to smart high school students) how we know what the structure of molecules looks like when they're so small. Thank you.

Answer 1

x-ray crystallography (there are many other ways)

http://en.wikipedia.org/wiki/X-ray_crystallography

Answer 2

We look - electron, x-ray, and neutron diffractions; microwave spectroscopy, infrared spectrometry, nuclear magnetic resonance spectrometry, mass spectrometry. All the various incarnations of the atomic force microscope. Field emssion microscope for atom positions on the tips of metal needles. Extreme electron transmission microscopy. Bunch of other stuff. No atom can escape visualization by itself or as a member of an array.

All results are consistent. Then comes theory of various capabilties to make predictions vs. observations. That also works to spec.

I have HyperChem-Lite. I crudely draw an organic molecule's heavy atom skeleton with bonding. I click "add hydrogens" and open valences are capped. I click "assign types" and all the atoms are properlyy hybridized. I click "minimize" and the thing jiggles and squirms until all the distances and angles yield the lowest energy lump. It's rude, crude, fast, and just about as good as a crystal structure.

With a little added sophistication we can do much the same to design pharma to do our bidding biologically. Stuff is as we expect it to be by observation, theory, and use all being consistent. Anybody with an empirically better idea is invited to speak up.

Answer 3

Well, most of it theory, for example the VSEPR (Valance Shell Electron Pair Repultion) theory can be used to hyphothize the shapes of the molecules. The VSEPR theory says the the bonding atoms or electrons will magnectially retract each other as much as possible to make a stable shape. I don't know if scientists have actually been able to view them, but it is possible.

Answer 4

With electron microscopes we can now see some molecular and atomic structures. Before that we could use the electron configurations to figure out how things would bond.