Calculating Particle Size from Molecule Weight

[mangetous]

I am looking to find a method for calculating the size of an element particle from its molecular weight. Is there any way of doing this? I have been trawling the internet for ages and I have found nothing yet :(

[jake-the-peg]

Not quite sure I know what you mean here

Are you talking about the size of an atom of a particular element from it's atomic number or mass umber?

You say molecule - do you mean that? you can't calculate the dimensions of a molecule from it's mass because you don't know it's structure or what atoms it is made of.

Perhaps if you gave us an example

[mangetous]

ok then maybe my jargon is wrong. I'm just asking for a friend who doesn't have internet access, and said the above to me!

Is it possible to calculate the size of an atom of a particular element from its mass number?

[fo3nix]

Are you looking for something like the semi-empirical mass formula?

http://en.wikipedia.org/wiki/Semi-empirical_ma ss_formula

What is this for?

Atoms are generally kind of similar in size. And even when they get much larger (the unstable ones), they're still usually very small on the length scales of the particular problem you're trying to solve, so can approximate them as being small.

[jake-the-peg]

Just to clarify this a bit the Semi-emperical mass formula relates to nucleii rather than atoms.

The size of an atom depends on the electron shells that are filled.

It tends to range between .1 and .5 nm

The precise size is difficult to calculate theoretically and there's a lot of quantum mechanics involved.

I think this has been done for simple atoms but only computationally (ie not from first principles) for heavier atoms.

There's something on it here

http://books.google.co.uk/books?id=Fb59En-lF9s C&pg=PA33&lpg=PA33&dq=calculating+electron+orb itals&source=bl&ots=Wc9jtauprJ&sig=5mIew7-QOf7 KVPxtiMh5w2oHOuM&hl=en&ei=zvauSb28O9yxjAepjYyn Bg&sa=X&oi=book_result&resnum=2&ct=result#PPA3 3,M1

There are some interesting side issues. I recall from a question on here some time ago that the reason Mercury is liquid is to do with quantum mechanical effects in the electron shells.