Consider a gas of identical boson-particles, e.g. atoms of a noble element. Let the gas be at a low temperature, s.t. the mean kinetic energy of the atoms is low. I think that during their collisions, the atoms can't get closer to one another than the van der Waals diameter (sum of the two van der Waals radii), because of the repulsion between the electron clouds. Am I wrong?
But, is the van der Waals diameter the true limit at which two atoms can approach one another? This diameter is of the order of a few Å, and I read that the minimal distance is a few nanometers, i.e. 10 times bigger.
Can somebody explain the situation?
Hi Sofia
So where did you read that minimal interatomic approach distances are around a few nanometers ( a few tens of angstroms)?
My theoretical chemistry is way out of date, but for near-equilibrium situations, calculating the potential energy curve for different interatomic distances would be my starting point.
If there's bond formation, then with the appropriate choice of method, look at the dynamics of vibrational states.
Incidentally, noble gases do form compounds, but the only noble-noble bond I've read about is in the compound Xe2Sb2F11 which contains a Xe–Xe bond, and this is supposed to be the longest element-element bond known (308.71 pm = 3.0871 Å), which is only a third of a nanometre.
Where bonding between non-ionic atoms occurs spontaneously, the atoms obviously need to be able to approach close enough to bond (assume no tunneling) e.g. two hydrogen atoms forming the singlet state (the triplet will be of higher energy, but I think even that doesn't start to experience repulsion any further out than about 3 angstroms - see diagram in link).
(I've omitted collision dynamics as you specify low K.E.)
Hope this helps - Paul
http://hydropole.ch/en/hydrogen/abouth2/
Dear Paul,
Where is the link of which you talk? I see no link.
Now, the gases that interest me are considered as non-interacting. They are monoatomic gases of identical atoms of noble elements, typically 4He.
I am not sure where I saw the distance of nanometers, I believe that it was in the reports of some experiments done at the Charles Fabry labs cca. 10 years ago.
You see, the distance at which atoms can approach one another depends on the kinetic energy of the atoms. At high temperatures this energy is high and can overcome part of the Coulomb barrier, coming close to one another up to some distance D. This distance is bigger than the sum of the radii of the two atoms, i.e. than the atom diameter, and it increases with the decrease of the temperature.
I would be glad to find a graph that shows such a dependence, but I have no idea where to look for it, and not even if such graphs are available.
Do you maybe know something in this direction?
My kindest regards,
Sofia
Hi Sofia,
In reply to your (separate) message re 4He sources, the best I can locate are as follows:
[EDIT ResearchGate seems to have uploaded links in a different order, as compared with text below]
1) A new interatomic potential function for helium (last link, Beck - seems to be available via ResGate)
2) (If it's still showing http://www.physics.udel.edu/~glyde/Pubs/116.pdf is probably not so useful (I'm not sure if I've deleted it) )
3) There appear to be many references at the end of Ab Initio Values of the Thermophysical Properties of ... (the google/books ref below), some of which might include what you're looking for
4) palesttras ref - may be of peripheral interest (He in condensed phase)
5) arXiv paper might also be of peripheral interest.
Hope this helps - Paul
http://www.tandfonline.com/doi/abs/10.1080/00268976800100381
http://www.physics.udel.edu/~glyde/Pubs/116.pdf
https://books.google.co.uk/books?isbn=1422318907
http://www.unicamp.br/~vitiello/palestras/gfmc40Talk.pdf
http://arxiv.org/ftp/arxiv/papers/1307/1307.4892.pdf
Dear Paul,
A lot of thanks for your effort. I requested the work of Beck, and I also looked in the other articles that you recommended.
To tell you frankly, I saw there a problem. In one of the articles, very detailed, I saw that at temperatures below 1ºK the 4He becomes liquid. I feel very confused, because the group that performed the experiments reported that they worked with a cloud of gas below the temperature of 1microK. I am looking for somebody who understands in such things.
Again, a lot of thanks for your kindness.
Sofia,
Thanks for your kind words.
Were you looking at the link below? That talks about a gas at 1microK. I'm absolutely not an expert in this field, so can't really add anything helpful.
There's a phase diagram for helium that shows up 3rd from left if I google:
helium gas liquid pressure temperature gas liquid
(I can't seem to copy the URL reliably)
It seems to indicate that helium (presumably helium-4) liquifies at 1.8 degreesK but doesn't show what happens as the pressure falls below about 0.015 bar.
However the Wiki page on Triple Points mentions that 4He is unusual " Helium-4 is a special case that presents a triple point involving two different fluid phases (lambda point).[1]"
Hopefully someone who is an expert will advise.
http://www.nat.vu.nl/~wim/Cold_Atoms/cold2.html
- Badges
- Science topic