<science talk>
This is true, however the atomic radius tends to decrease as the atomic number increases, until you get to the Alkali Metals (the first column in the periodic table) where the radius suddenly increases again, i.e. O is smaller then N, N is smaller then P, and NE is smaller then NA. Periodic Table.

This has to do with the relative electronegativityof the elements, an element that is more electronegative attracts the electrons more strongly then one with a weaker electronegativity.

That being said, we are dealing with N2 and O2 here, not N and O. The covalent radius of Oxygen is 73 pm, and the covalent radius of Nitrogen is 75 pm, so even in a covalent configuration the Oxygen is smaller then Nitrogen.

The additional problem with Oxygen is that it can Oxidize the rubber on the interior of the tire, being used in the process and therefore reducing the pressure inside the tire faster then Nitrogen would (theoretically at least, I do not know of any studies that have shown this to be a major effect).
</science talk>

~Droid

 

Atomic mass has little to do with molecular size. Its all about valence states and bond characterization. In this case, it just so happens that the N2 nolecule has a diameter ~218pm vs O2 with ~240pm. (http://www.webelements.com/ )

N2 actually IS smaller than O2, but this is because the triple covalent bond in N2 being "tighter"than the double bond in O2. The advertising claims that N2 is larger; this is inappropriately based on the VanderWaal radius, which is not a factor here.

edit:
as droidicus' response differs, this illustrates the ambiguity in HOW sizes are determined at such a small level. Different radii, different method.

FWIW, the covalent radius assumes a SINGLE bond; the 'bond length' radius I used is taken from the natural molecule; IMHO (not being a chemist) this seems more correct in some situations.

I might also be wrong about the N2 triple bond?

 

That's interesting... it makes sense that atomic radius would tend to decrease as the size of the nucleus increases. But even if droid were right in that N2 is larger than O2, would that seriously have a measurable impact on how much leaks out of a tire?

 

doubt it very very much. I posted somewhere in the past to the effect that the statistical distribution of pore sizes in the tire is probably wide enough and the difference between O2 and N2 sizes small enough such that both will probably pass through with the about the same ease (difficulty). Particularly on the time scale that a driver should be checking their tire pressure, anyway.

the overwhelmingly dominant reason for using N2 is that it is water-free as a result of its production process; water being bad for several reasons. Oh yeah, and becasue using N2 lines someone's pocket with more $$.

OK. enough N2 talk- there are other existing threads for this. Back OT...

 

First of all, nitrogen is not an inert gas. There are many compounds, including NO2 in vehicle emissions, that contain nitrogen. (Ever heard of nitroglycerin?).

Moisture is just another gas in the mixture, which happens to be H2O in gaseous form. The problem with H2O in our typical ambient temperature is that it tends to change into liquid form (water). When it does so, its volume changes dramatically, significantly affecting the air pressure in the tire. Nitrogen and oxygen both liquify at very very low tempratures, so their change of state is not an issue.

I believe one good reason for using nitrogen in race cars is that it doesn't contribute to fires. If a tire catches fire, the oxygen in the tire could contribute to that fire, while a tire filled only with nitrogen would not burn as easily. A tire that blows out would shoot a burst of nitrogen out, possibly even putting a fire out by choking it. That may also be a good reason for filling electrical conduits with nitrogen - fire prevention. It's also true that oxygen is corrosive - it combines with metals and oxydizes them, while nitrogen doesn't.

 

Well... actually even though it's not among the typical group of inert gases, Nitrogen is pretty inert. Inert does not mean that there's nothing harmful about things that contain it, rather it means more about whether the gas has readily reactive properties of its own, which nitrogen has not. This is exactly what makes Nitrogen inert. By itself it is very safe.


As for vapor, by using a pure bottled gas you are ensuring that there's no water vapor in it as opposed to using regular compressed air. Using a pure gas ensures that it can not combine with other components to create unwanted by-products like vapor. Once you decide to use a pure gas you need to pick a safe one, non flamable, non corrosive, etc. Nitrogen is one of the safer ones and it's cheaper to make than others because there's lots of it in regular air. We could just as easily use Helium or something else, but since Nitrogen makes up 75% of normal air, it is the cheapest to extract.

As for the electrical issue, it's definitely all about keeping water vapor out of the cable. In a coaxial cable, the critical property that makes it work is that there are two conductors (center conductor and the shield) separated by a non conductive dielectric material. The actual signal doesn't travel through the center conductor as you might think, it actually travels as an electric field between the center and shield - traveling through the dielectric material. Many of these cables do not have solid foam dielectric like your cable tv wire, they simply have empty space (maintained by nonconductive spacers to keep them aligned properly). If that space has water vapor in it (remember water is conductive) the electric field inside the cable will be inconsistent or umpredictable as it changes states. Maybe only slightly, but over a thousand feet of cable running up that tower small amounts become significant. So, replacing the air in the coax with nitrogen ensures a consistent vapor free gaseous dielectric.

 

Hmmm ... that would technically make our wheels lighter

I would go for that, and the more you overinflate cold pressure, the lighter the wheel, and since it is moisture free, you could essentially put it closer to the max hot pressure of the tire. ... now to find out what the max hot pressure is and to get some Helium

But wait ... isn't Helium flammable

I was trying to find out what pressure to put my tires at so I thought I would pull this thread out of the graveyard

guess I will go with 55 psi and see how I like it

 

look on the sidewalls for a starting point.

 

You have BOTH? Are they both just for you or is one for a family member?

Lucky dog, you get the best of both worlds.....

 

I thought helium was an inert gas .... hydrogen is flammable for sure.