Re: [Phys-l] size controversy

[John Denker <jsd@av8n.com>]

On 12/03/2009 10:15 AM, Bernard Cleyet wrote:

> Which has a larger atomic radius H or He?  The question relates to  
> leakage in spectrum tubes.  

That's two different questions.  Size by itself does 
not determine diffusion rates.

As to the question about leakage / diffusion, the
answer very much depends on what the tube is made of.
You can easily do the experiment to confirm that He
diffuses out of a latex balloon much faster than H2
does.

At the other extreme, hydrogen diffuses through metals
incomparably faster than helium does.  If you want 
to think about this in terms of size, which I don't
necessarily recommend, consider that the H+ ion has
a reeeally small ionic radius ... and H+ is what is
diffusing through the metal.

If the question really is about size per se, that is 
usually a tricky question, since size is a classical
concept that doesn't necessarily apply cleanly to
atoms.  Even classically, it is sloppy to compare
the "size" of objects that are not all the same shape.
However, in this case, a simple physics argument 
suggests that H2 must be bigger than He.  They are 
isoelectronic!  And the positive charge is a little 
more spread out in H2.  Imagine starting with H2 and
somehow forcing the protons toward each other.  The
molecule will get smaller.  When the protons get to
the same place, you have made a peculiar unstable
isotope of He.