Distances between Atoms in a Gas Molecule

[dingsbunnyranch]

How do you calculate the distances between atoms of a given gas molecule? The reason I am asking this is so I can find the moment of inertia of certain molecules so i can determine degrees of freedom for a gas at a certain temperature. Is there an easier way to determine them?

 

[Pythagorean]

How do you calculate the distances between atoms of a given gas molecule? The reason I am asking this is so I can find the moment of inertia of certain molecules so i can determine degrees of freedom for a gas at a certain temperature. Is there an easier way to determine them?

can't gas expand and compress? Wouldn't this imply that there is no absolute distance between atoms in a gas, and that its relative to the pressure and volume?

Just using logic, with no advanced physics training (i'm just starting my third year as a physics undergrad), I'd guess that you could use the ideal gas law:

PV = nRT where P is Pressure, V is volume, n is number of moles, R is the gas constant, and T is the temperature.

you could find out the volume per atom (V/n = RT/P) and derive it from there, i suppose. But I think gases rely on fluid dynamics, which are really complex, and the distance between atoms will be different for different localities within a given 'cloud' of gas.

 

[HallsofIvy]

Pythagorean, I think you misunderstood the question. The question was not about the distance between molecules (atoms) in a gas but about the distance between atoms in a single molecule.

I don't see any simple way to answer that- it depends on the type of gas. You might be able to look up distance between atoms for a particular molecule in something like the CRC tables.

 

[Pythagorean]

Pythagorean, I think you misunderstood the question. The question was not about the distance between molecules (atoms) in a gas but about the distance between atoms in a single molecule.

I don't see any simple way to answer that- it depends on the type of gas. You might be able to look up distance between atoms for a particular molecule in something like the CRC tables.

ah, yes, I see. I think this is a common study in chemistry.

from wikipedia:

This weak attraction takes place already at long distances of 100 nanometer, whereas the stronger repulsion only becomes preponderant at distances of 100 picometer, a distance which is similar to atomic radii. The atoms are held together at the distance at which the attractive and repulsive forces cancel out.

The length of a chemical bond is taken to be the average distance between the two nuclei. For gases it can be determined to a high degree of accuracy by rotational spectroscopy or by electron diffraction. For solids, it is usually measured by X-ray diffraction: however this method cannot measure the length of bonds involving hydrogen, for which neutron diffraction must be used. Average bond lengths (often with standard deviations) for a large number of bond situations in inorganic and organic compounds have been calculated from data in the Cambridge Crystallographic Database. Other factors being equal, a shorter bond is also a stronger bond.

http://en.wikipedia.org/wiki/Chemical_bond/Temp