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Comment by ThePhysicist | Hacker News Reader

Helium is very good at getting through the tiniest cracks due to its small atomic diameter. One uses it to find leaks in devices with ultra-high vacuum seals (e.g. in low-temperature physics) by attaching a pump with a simple mass spectrometer sensitive to Helium to the device under test and then using a small helium gun to test different parts of the device from the outside. When spraying it at the leak some of it is pushed inside by the pressure difference and can be detected by the pump spectrometer.

Hydrogen would be even better of course but it is rather dangerous when mixed with air.

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Hydrogen would not be better. A helium atom is considerably smaller than a hydrogen molecule. Even if you could use atomic hydrogen, the helium atom's radius is significantly smaller due to the higher central charge that is only partially screened by the "other" electron.

Hydrogen binds with other hydrogen atoms to create H2 which is the only form of elemental hydrogen you’ll see outside of a lab helium is unique in that its singular atom is its own elemental molecule.

It's not unique in that. There are other single-atom molecules.

It's not unique. It's just the smallest.

That point has already been well-established in all the other comments.

Im guessing it's the only one that's gaseous at room temperature and pressure?

Edit: Never mind, all the nobel gasses.

It’s not all the Nobel gases, elemental Xenon is Xe2, so are the rest I think they are also not that Nobel anymore everyone of them besides helium was found to form compounds.

Edit: apparently even helium can form compounds it’s just freaking hard to force it to play nice with others - https://en.m.wikipedia.org/wiki/Noble_gas_compound

Xenon is usually found in atomic form. Xenon dimers are not generally found naturally.

At the risk of sounding fussy, and hopefully just to inform, these are called "noble gases", and it isn't capitalized (except of course at the beginning of a sentence). No connection with the Nobel Prize or anything else with that name.

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

Noble, like nobility, not Nobel, like the Swedish chemist.

Right. "Noble" is also used for the noble metals -- gold, platinum, etc., which are relatively non-reactive.

Huh. I would not have guessed Xenon formed dimers at all, but I'm still skeptical that's its usual elemental form...

nit-pick ... Noble Gas, not "Nobel Gas"

TIL, which others? I thought other noble cases are dimers like Xenon also unlike helium I think all of them now found to form some compounds especially with the fluoride family.

All noble gases exist as monoatomic elements.

They don't form compounds easily but there exist molecules containing at least xenon, radon. Even helium can be made to participate in reactions if pressure is high enough.

You only tend to get a noble gas reacting with something if you approach the arena of irresistible force (fluorine) meets unmovable object (xenon), and get Xenon Tetrafluoride. https://en.wikipedia.org/wiki/Xenon_tetrafluoride

The noble gases have complete electron shells and are quite satisfied with themselves, not needing any bonds with anything else. However, the larger the atom, the more flexibility it has with how many electrons it can hide down the back of the sofa. Xenon is the largest non-radioactive noble gas, so it was the first to have a noble gas compound discovered. Fluorine is just desperate to acquire an electron to complete its electron shell, and will do it by whatever dirty tricks it can manage, which is why it is able to wrest one from Xenon's tight grasp.

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

This sort of thing (along with Derek Lowe's blog) makes chemistry interesting for me.

Derek's blog is absolutely incredible. For the uninitiated, I suggest to start with the "Things I Won't Work With" category:

http://blogs.sciencemag.org/pipeline/archives/category/thing...

FOOF!

I don’t know anything about chemistry beyond high school (US public school) but presumably helium is the smallest single-atom molecule you’ll encounter if hydrogen is off the table.

Hydrogen would be better. Size isn't the most important factor. Gasses with low molecular mass have higher particle velocities. The velocity affects mixing, passage through holes (including filters), and the speed of sound. Hydrogen is about 2 AMU, 1 for each atom, while helium is about 4 AMU.

You can find the formulas in a typical college chemistry textbook.

If "hydrogen would be better", I guarantee companies that make highly-sensitive cryostats (like companies that manufacture MRIs) would do it.

They don't - even knowing He is more expensive that H, because helium is better at leak testing than hydrogen is

It gets worse than that when your cryo equipment has a “cold” helium leak. Ie, something leaking on the dilution fridge only at helium temperatures (4.2K) and lower, but no trace of a leak at room temp.

Repeated bouts of guessing where the leak might be and trying to plug it (usually by welding or re-machining some part), cooling down, checking if it worked, warming up, ripping some more of your hair out, repeat.

Yeah I had that as well, in that case it was best to replace all the Indium seals right away as like you said it takes 24 hours to do a full cooldown/warmup cycle anyway, so spending a day to make new seals is often faster. That's a part of low-temperature physics I definitely don't miss!

I worked on a system where I needed to reach and maintain pressures of 10^-3 tort, and that was hard enough. I can’t imagine having to deal with helium sensitivity.

10^-3 tort is very low. That will cause lawyers to explode.

Just having fun with your typo...

It's why companies that do stuff with ultra-low-temperature pressure vessels typically dramatically overengineer the devices.

Electronic parts also get helium exposure on the launch pad in a rocket. I know for example there can be helium sensitivity issues on gyros for CubeSats.

Wouldn’t hydrogen only work if it was atomic hydrogen? I thought molecular hydrogen gas was larger than helium.

As a bonus hydrogen is higly explosive, helium is not.

Hydrogen is no more explosive than other flammable gasses when mixed with ambient air. To get the explosive effect, you need to mix it with oxygen in the right proportion (2:1, molecule-wise) and not have any filler gas such as nitrogen.

One of the dangerous things about hydrogen is its wide flammability range: from 4% to 75% concentration of hydrogen in air. Hydrogen will detonate in concentrations between 18% and 59%. In comparison, gasoline has a surprisingly small flammability range of 1.4% to 7.6%.

What drives this wide range of flammable concentrations? Is it just an issue of low activation energy?

Also the upper end is impacted by the low number of oxygen atoms each hydrogen molecule can react with, in this case it's just one, compared to gasoline with say 8 carbon and 18 hydrogen atoms which bounds to 25 oxygen atoms. Mind you, the size of a gas molecule doesn't affect how many you get per volume at a certain pressure and temperature. So 50% gasoline vapors and 50% hydrogen are both molecule counts, with the former requiring 25 times as much oxygen for full combustion, compared to the latter.

Hydrogen exists as H2 in its elemental form you’ll you’ll need to ionize it to separate it and keep it in an ionized or other way isolated form to prevent it from binding to other hydrogen atoms.

If you are talking about ionized hydrogen, you are no longer talking about a gas. If it’s ionized, it is a plasma.

And if it's a plasma...it's gonna melt whatever you're trying to leak-check :)

Hydrogen is soluble in most metals. It will just diffuse right into it, like air flowing through a foam filter.

Added bonus: if the metals in question form chemical bonds with hydrogen, (titanium in particular) it will happily do so, even if it's deep inside the metal. The metal will turn very brittle very quickly if it's exposed to molecular hydrogen.

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

Even a single hydrogen atom is bigger than a single Helium atom. The charge of the helium nucleus is higher and thus "pulls" the two electrons further inward than Hydrogen's single proton pulls its single electron.

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