1. Aug 25, 2008

### jenko4292

I'm studying A2 (second year college) physics and im supposed to be finding out about the size of an atom and how it was figured out but I'm having problems because of the fact that nobody seems to know. I realise it is different for each atom/ion but how could it be figured out?
Maybe it's a trick question.. it's different every time and immeasurable due to the nature of electrons... hmmm?

2. Aug 25, 2008

### granpa

I tried approximating the van der waals volumes from the density of the pure elements. but when I tried using the numbers to calculute the volume of simple covalently bonded molecules like simple oils the results didnt work out.

If you have better luck I would like to hear about it.

volume (cubic angstroms) = Atomic weight * 1.66 / density (grams/cubic centimeter)
density (of molecule in g/cc) = sum of atomic weights * 1.66 / sum of volumes ( cubic angstroms)

I dont claim this to be anything more than a first approximation and you should check my math. I have been known to make mistakes.

3. Aug 25, 2008

### jenko4292

So noone really knows?
I'm just looking into the history of the physics to be honest.

4. Aug 25, 2008

### Staff: Mentor

Why does one say that no one knows?

X-ray diffraction is one method used to look at the lattice parameters of crystals of elements and compounds. From that we can infer the size of an atom.

Since the atomic radii (diameters) have been tabulated, someone obviously has an idea of the size of an atom. And yes - it does matter as to the form (element or compound).

And example - http://www.webelements.com/hydrogen/atom_sizes.html

We know the size of atoms well enough to use that information.

Now there are "atomic force microprobes" that look at the characteristics of individual atoms.

5. Aug 25, 2008

### jenko4292

I think I must have worded this incorrectly I was just interested in knowing the history of these X-ray scattering experiments like who first did them, why they thought it would work. When they did it. and why the constant movement of electrons doesnt effect the results.

6. Aug 26, 2008

### Staff: Mentor

Here is some background on X-ray crystallography and determination of atomic size (radii).

University of Wisconsin (Madison) Library - Subject Guide: X-Ray Crystallography
http://chemistry.library.wisc.edu/subject-guides/x-ray-crystallography.html [Broken]

Crystallographic Education Online
http://ww1.iucr.org/cww-top/edu.index.html

An Introductory Course by Bernhard Rupp
http://www.ruppweb.org/Xray/101index.html

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

http://en.wikipedia.org/wiki/Atomic_size