atomic radius of He compared to H2

March 27, 2007 6:46 PM

how does the atomic/molecular radius of hydrogen gas molecules (H2) compare to that of helium gas (He)?

for instance, the atomic radius of He is 31 pm.

it's fairly straightforward to get the atomic radius of a hydrogen atom - but not the H2 molecule - from this site and others.

and what i want to know exactly is, at standard atmospheric conditions what is the radius for He gas and what is it for H2 gas?

i'm trying to compare the escape-rates of these two gases out of mylar/aluminized balloons over time (which would presumably be slower for H2 if the molecular radius is greater).

posted by jjsonp to Science & Nature (6 answers total)

The bond length of H2 is pretty short. If you can't find a value for the bond length, you could probably approximate the bond length as being an overlap of 0.5* the atomic radius, or thereabouts, and get a dimension from that.
posted by janell at 7:05 PM on March 27, 2007

I would think that the radius is much less important than the mass; since hydrogen weighs half as much as helium, the hydrogen will be moving (and diffuse) 1.4x as fast.

If you still want radii, googling "Molecular radius" and H2 will give you some data.
posted by cgs06 at 7:09 PM on March 27, 2007

My chemistry is from back in the day, but it seems to me that it would be the covalent radius of hydrogen which Wikipedia gives as 37 pm
posted by Neiltupper at 7:13 PM on March 27, 2007

My friend says: Just use graham's law, r1/r2 = sqrt(MM2/MM1), the escape rate would be about the same.

Maybe someone can verify this?
posted by theiconoclast31 at 7:54 PM on March 27, 2007

The dependence of escape rate on particle size, particle shape, and particle mass will not be simple. Take care.
posted by hAndrew at 9:04 PM on March 27, 2007

Best answer: The radius is what's important, and that's why the best leak testers use helium instead of hydrogen, even though helium is starting to get expensive. The radius matters because there are a lot of holes that are too small for hydrogen to pass through which helium goes through just fine. If the hole is too small, it doesn't matter how fast the hydrogen molecule is going or how light it is; it's still going to bounce.
posted by Steven C. Den Beste at 9:15 PM on March 27, 2007