Gas questions and Bose-Einstein condensate

[Guest gamemastermh]

I was reading one of Asomov's many collection of essays of chemistry (Asimov of Chemistry, to be exact) and I thought of a few interesting questions I thought would be interesting to post

 

First: The distance between gas molecules can be derived from many factors (you all know how to do this I'm sure). So, when the gas is frozen and becomes a solid at low temperatures, how is the atomic radii found. With all the molecules atracted to each other (usually attraction is the sharing of electrons or charge) how do chemists really know that the molecules are touching at just the outer shells?

 

Second: If one were to find the atomic radii from Helium this way (described above),which has a freezing point of O degrees Kelvin, how is it possible that a Bose-Einstein condensate is not formed, changing the characteristics of those helium atoms. If this happens, the atomic radii that would be derived would have to be innacurate.

 

Does anyone out there know the answer to my quandries?

[VendingMenace]

how do chemists really know that the molecules are touching at just the outer shells?

 

Well, then. I think we could prolly spend a long time discussing just this question. The short answer is; we don't. and we don't assume that just the outer shells are touching.

 

You see, electrons occupy orbitals whose radii are not very well defined. Not only are there complications for atoms with more than 1 electron that render it impossible for us to find out what the radii would be, but even if we could figure out exactly what the orbitals look like, we would still not really be able to say where the "outer shell" ends. This is because the orbitals are not a discription of where the electron is, but where it is likely to be. That is, given an orbital we can say "we will find an electron in this volume 95% of the time." So it is really difficult (if not impossible) to define where the outer shells lie.

 

As far as the calculations you are reffering to go, the radii that is being computed is really a kind of effective radii. It lets us know about how close together the atoms like to get. It is just a nice number to know when we are trying to figure out how individual molecles interact, but that is about it. It is still pretty cool stuff though. But the take home lesson i guess is this, electrons do not have fixed postions, they are goverened by probabilities, and as such talking about definite position values is not really the most correct way of thinkinng about them. However, even though we know it is wrong, we can think about things this way and still get some usefull values. In this case, about how far apart molecules like to be

[Guest gamemastermh]

Well, I think that answers my question almost totally. What I really would like to know is, at absolute zero, don't electrons and all subatomic particles stop moving? So, theoretically, there should be a very defined outer electron shell. I know a Bose-Einstein condensate forms when some elements reach absolute zero, so is there any possible way to get frozen helium? Helium has a freezing point at almost absolute zero.

[VendingMenace]

What I really would like to know is, at absolute zero, don't electrons and all subatomic particles stop moving?

 

Nope. At absolute zero all motion does not stop. This would violate the generalized uncertainty priciple, from wich we can find that we can not simultaneously find the position and momentum of a particle to an arbitrary degree of presision. That is to say that we cannot know exactly where something is and how fast it is moving. If all motion stopped, then we would know both where it is and how fast it was moving. Thus, at absolute zero electrons still move about and molecules still vibrate.

 

What is really happening at absolute zero is this; molecules (and atoms) are at their lowest allowable quantum states. Suprizingly that means that we know everything we can know about something, quantum mechianically speaking, when it is at absolute zero. We know, without measuring, what state it is in. This is really cool. It means that everything is perfectly ordered. And if everything is perfectly ordered, then there must be no entropy at absolute zero. Its really quite wild!

 

So, no motion does not stop, but rather, things reside in the lowest possible energy state.

 

 

I know a Bose-Einstein condensate forms when some elements reach absolute zero

 

I hate to be a nit-picker, but we have never reached absolute zero. So, anything we might know about behavior at absolute zero is quite theoretical. That is not to say it is wrong, it is just that we cannot reach absolute zero.

 

so is there any possible way to get frozen helium

 

I don't see why not. I, personally dont know how to do it, but i dont know any reason why it wouldn't be possible.

 

A long time ago, i remember reading something about people trying to make solid hydtrogen. Basically, the cooled down a sample of hydrogen and then shot a projectile from a rail run at the sample. The hope was that the pressure from the impact of the projectile would force the hydrogen into its meltallic state. I foget if it worked or not (i think it did, but i was like 12 at the time, so i don't really remember), but i could imagine it might work for helium as well.

 

But all that is kinda speculation anyways, as i don't really know alot about that stuff.

 

Anywyas, hope that helps, ask more questions if you got em