# Atomic Volume Question

1. Mar 31, 2009

### Faultline369

Hi!

I am new here, and I am no professional physicist, but I love science and physics. I don’t claim to know much at all, but I hope to someday. Nor am I a math wiz, in fact, I struggle with it.

With that said, I was wondering if a percentage of the reciprocal of the “Atomic volume” is the number of moles of atoms per cm3.

For example: Lead’s atomic volume is 18.272 cm3/mol. It is FCC packed, so there is actually only 74% lead and the rest is void. Therefore: 0.74 * (1 / 18.272) would equal the number of mols of just Lead atoms in One cm3 . . . right?

So multiplying that above answer by Avogadro’s# would then give the “total number of individual lead atoms” per cm3? . . . right?

Then dividing 1 cm3 by this “total number of individual lead atoms” would then yield the volume of One single atom??? (this would be a reciprocal again)

If this is wrong, could you take a second and show me (if possible) how to get individual atom volume from atomic volume?

THANKS!!

2. Mar 31, 2009

### malawi_glenn

well IF atoms HAD individual volume, your method is the one that we would use, but now atoms does not have a definite volume due to their quantum mechanical nature.

3. Mar 31, 2009

### alxm

Well you can do something like that, Faultline.

The problem malawi_glenn alludes to, is that the question is what that number would actually mean?
Atoms aren't tiny hard spheres. They don't have a well-defined 'radius'. So how would you define it?
The distance from the nucleus that has the highest electron density?
The distance from the nucleus that encloses a certain percentage of the electron density?
Half the distance to the closest atom?
Etc.

Even if you chose a certain definition, the radius would not be constant. A metal atom in a metal crystal would have a different radius than a metal atom in a ceramic material which would be different than the radius for the metal atom bound to some molecule, which would be different than the metal atom's radius in its different ionized states.

That isn't to say the number is useless though. It does say something about the 'size' of the atom, and it's on the same order of magnitude as the other numbers. But that's also about it.

4. Apr 1, 2009

### Faultline369

Ahhh, I see what you mean . . . and I agree!

But at least now I know I was doing the figuring right! Thanks for the responses!