Sometimes, definitions aren't the most mechanically sound constructs available.

Very few air seals protect against helium, the smallest gas. Most materials, even metals, are like sponges to helium. It'll seep through slowly.

I was about to correct you and say that hydrogen is the smallest, but apparently, hydrogen's atomic radius is actually a bit bigger. Helium's radius is smaller due to the larger charge of the nucleus making for a tighter electron cloud.

I too had the same thought, and then thought maybe hydrogen molecule is being considered. Read up to see that hydrogen atom has bigger radius. TIL.

Hydrogen creates molecules H2.

Yup. Larger atomic radius due to less nuclear charge, and it's only stable in molecular form - H2.

Helium can cause problems with the metals containing fusion reactors - He bubbles can get inside the metal structure and weaken it considerably:

"The sun makes energy by fusing hydrogen atoms, each with one proton, into helium atoms, which contain two protons. Helium is the byproduct of this reaction. Although it does not threaten the environment, it wreaks havoc upon the materials needed to make a fusion reactor.

"Helium is an element that we don't usually think of as being harmful," said Dr. Michael Demkowicz, associate professor in the Department of Materials Science and Engineering. "It is not toxic and not a greenhouse gas, which is one reason why fusion power is so attractive."

However, if you force helium inside of a solid material, it bubbles out, much like carbon dioxide bubbles in carbonated water.

"Literally, you get these helium bubbles inside of the metal that stay there forever because the metal is solid," Demkowicz said. "As you accumulate more and more helium, the bubbles start to link up and destroy the entire material.""

https://phys.org/news/2017-11-fusion-energy.html

Case in point, helium filled balloons that deflate over a couple days.

That’s just because rubber balloons are extremely porous, especially when stretched. They don’t hold other gases well either.

Helium balloons are commonly made out of Mylar instead of rubber, and can keep inflated for weeks.

The type intended for keeping around / in a shop for longer time are, but the children's party type often are inflated on the spot.

Completely irrelevant, yet amusing: At a former employer, we had access to what can only be described as unlimited supplies of hydrogen.

This led - in addition to lots and lots of eardrum-splitting workshop pranks - the occasional kids' party balloons being inflated with hydrogen. (As it was free, which He definitely isn't. HS&E be damned.)

A then colleague of mine claimed they'd cracked a window and caused instant panic at his son's birthday party when a bright spark decided to put a balloon to a lit candle to show the other kids it'd pop.

I doubt the cracked window (given a standard party balloon is pretty small - and a cracked window would likely mean lots of -ahem- eardrum deficiency in the assembled crowd) - but I can imagine it got the kids' attention all right when the balloon more or less exploded.

(We did crack a couple of windows in the workshop, though - you'd be amazed at the bang a litter bag filled with a hyd/ox mixture can produce if set alight.)

Hydrogen alone just makes a soft pop; as you say, for a bang it needs to be mixed with oxygen prior to ignition.

>you'd be amazed at the bang a litter bag filled with a hyd/ox mixture can produce if set alight.

Try a large garbage bag with oxyacetylene mix (do not try this at home).

I guess - never having tried the party balloon trick myself - that such a small amount of hydrogen will mix well enough with the surrounding air as the balloon pops to create a (modest, at least) bang as the mix ignites, but this is just conjecture.

Maybe I'll have to do some empi research one of these days - we do have a few bottles of oxygen and acetylene in the workshop at my current employer.

Hey ho, time for an engineer to pay our welders a visit.

Safety first!

Air filled balloons also deflate over a couple days.

Compare with aluminium coated foil. They don't deflate that fast.

Or intestines of cattle.

https://en.wikipedia.org/wiki/Zeppelin

Early Zeppelins used rubberised cotton for the gasbags, but most later craft used goldbeater's skin, made from the intestines of cattle

I thought hydrogen was the smallest gas, not helium.

Hydrogen normally comes as a molecule (H2), not an atom.

Correct, but also just as importantly the He atom has both a higher nuclear charge and a full valence shell, which pulls the electrons in more tightly. I’d have to look it up but I believe a single He is even smaller than atomic hydrogen or at least comparable.

Edit: looked it up and yes, helium is smaller than atomic hydrogen.

Depends on your definition of smaller, but apparently it's not clear-cut: https://www.quora.com/Which-element-is-bigger-an-atom-of-hyd...

Well, air is also compound of rare gases of which Helium is one.