While I was looking at the periodic table today, I realised that there were gases that were much lighter than helium such as hydrogen. If hydrogen is lighter than helium, why do we insist on using helium in balloons?
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6$\begingroup$ For some applications, $\mathrm{H}_2$ is more appropriate. For example, weather balloons often use hydrogen due to cost, with side benefits being lower density and lower diffusion rates across the membrane; flammability is not a major concern. $\endgroup$– 200_successCommented Sep 8, 2014 at 17:40
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2$\begingroup$ Since helium is on shortage, fewer and fewer places use it any more for balloons. $\endgroup$– GregCommented Sep 10, 2014 at 3:07
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$\begingroup$ Using H2 is needed for balloons too, because the Hydrogen in normal conditions exists only as paired atoms in an Hydrogen molecule. This is important for example for the diffuse rate - where Helium has much higher rate as H2 - exactly because the Hydrogen molecule (H2) is much bigger as one atom He - so the Helium has higher diffuse rate as H2. $\endgroup$– clt60Commented Sep 10, 2014 at 10:03
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6 Answers
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As other answers have noted, the only gas lighter than helium is hydrogen, which has some flammability issues that make it more difficult to handle safely than helium.
Also, in practice, hydrogen is not significantly "lighter" than helium. While the molecular mass (and thus, per the ideal gas law, the density) of hydrogen gas is about half that of helium, what determines the buoyancy of a balloon is the difference between the density of the gas inside the balloon and the air outside.
The density of air at STP is about $\rho_{\ce{air}}=\pu{1.2754 kg m-3}$ , while the densities of hydrogen and helium gas are $\rho_{\ce{H2}}=\pu{0.08988 kg m-3}$ and $\rho_{\ce{He}}=\pu{0.1786 kg m-3}$ respectively. The buoyant forces of a hydrogen balloon and a helium balloon in air (neglecting the weight of the skin and the pressure difference between the inside and the outside, which both decrease the buoyancy somewhat) are proportional to the density differences $\rho_{\ce{air}} -\rho_{\ce{H2}}=\pu{1.1855 kg m-3}$ and $\rho_{\ce{air}} -\rho_{\ce{He}}=\pu{1.0968 kg m-3}$. Thus, helium is only about $7.5\%$ less buoyant in air than hydrogen.
Of course, if the surrounding air were replaced with a lighter gas, the density difference between hydrogen and helium would become more significant. For example, if you wished to go ballooning on Jupiter, which has an atmosphere consisting mostly of hydrogen and some helium, a helium balloon would simply sink, and even a pure hydrogen balloon (at ambient temperature) would not lift much weight. Of course, you could always just fill the balloon with ambient Jovian air and heat it up to produce a hot hydrogen balloon (not to be confused with a Rozière balloon, which are used on Earth and have separate chambers for hot air and hydrogen / helium).
Ps. A quick way to approximately obtain this result is to note that a hydrogen molecule consists of two protons (and some electrons, which have negligible mass), and thus has a molecular mass of about $\pu{2 Da}$, while a helium atom has two protons and two neutrons, for a total mass of about $\pu{4 Da}$.
Air, meanwhile, is mostly oxygen and nitrogen: oxygen has a molecular mass of about $\pu{32 Da}$ (8 protons + 8 neutrons per atom, two atoms per molecule), while nitrogen is close to $\pu{28 Da}$ (one proton and one neutron per atom less than oxygen). Thus, the average molecular mass of air should be between $28$ and $\pu{32 Da}$; in fact, since air is about three quarters nitrogen, it's about $\pu{29 Da}$, and so the buoyancies of hydrogen and helium in air are proportional to $29 - 2 = 27$ and $29 - 4 = 25$ respectively. Thus, hydrogen should be about $\frac{(27 - 25)}{25} = \frac{2}{25} = \frac{8}{100} = 8\%$ more buoyant than helium, or, in other words, helium should be about $\frac{2}{27} \approx 7.5\%$ less buoyant than hydrogen.
Pps. To summarize some of the comments below, there are other possible lifting gases as well, but none of them appear to be particularly viable competitors for helium, at least not at today's helium prices.
For example, methane (molecular mass $\approx \pu{16 Da}$) has about half the buoyancy of hydrogen or helium in the Earth's atmosphere, and is cheap and easily available from natural gas. However, like hydrogen, it's also flammable, and while it's somewhat less dangerous by some measures (burn speed and flammability range), it's more dangerous by others (total energy content per volume). In any case, the reduced buoyancy, together with the flammability, is probably enough to sink (pun not intended) methane as a viable alternative to helium.
A much less flammable choice would be water vapor which, with a molecular mass of $\approx \pu{18 Da}$, is only slightly less buoyant than methane at the same temperature and pressure. The obvious problem with water is that it's a liquid at ambient temperatures, which means it has to be heated to make it lift anything at all. This wouldn't be so bad (after all, you get extra lift from the expansion due to heat), except for the fact that it makes any failure in the heating system a potential disaster — whereas a hot air balloon will just gently drift down if the burner fails, a hot steam balloon can experience catastrophic buoyancy loss if the vapor condenses.
Despite these drawbacks, hot steam balloons have certainly been suggested, studied and tried in the past — alas, not always particularly successfully (although, apparently, there have been much more successful attempts as well). There are various ways in which the condensation issue could potentially be reduced, such as adding extra insulation layers to the balloon envelope, or even surrounding the steam balloon with a more conventional hot air envelope. So far, however, it seems that steam balloons remain firmly in the realm of nifty but impractical ideas.
Other potential lifting gases, with molecular mass similar to methane and water, include ammonia and neon. Neon, being a noble gas like helium, would certainly work and be safe, but alas, it's both less buoyant and more expensive than helium.
Ammonia, on the other hand, while much less flammable than methane, is rather toxic and corrosive (not to mention really stinky, which, given its other properties, is probably a good thing). I don't think I'd like to fly in an ammonia balloon, but apparently, some people do! It seems that its main advantage (besides being much cheaper than helium) is its relatively low vapor pressure, which makes it easier to store and handle in compressed form.
Thus, at least for some niche applications (mainly hobbyists and some weather balloons, AFAICT), ammonia might actually be the most viable alternative to helium (and hot air) today, with methane / natural gas perhaps coming second. If helium were to become more scarce and expensive, these low-cost lifting gases (and possibly other alternatives, like helium recovery or even steam balloons) might become more practical. Then again, so would hydrogen — its safety issues, though well known, are not insurmountable, especially not for things like unmanned weather balloons where the risks are much less.⠀⠀⠀⠀⠀⠀
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$\begingroup$ Excellent answer. The only thing I have to add is that methane (MW 16)has about half the buoyancy of helium (29-16=13), so if you REALLY don't care about safety you can fill your balloons with that. (Natural gas is a whole lot more readily available than hydrogen.) Hydrogen is more dangerous than methane by some measures (burn speed, flammability limit) but methane is more dangerous by others (total volumetric energy content.) $\endgroup$ Commented Sep 8, 2014 at 13:14
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$\begingroup$ I'd never heard of Roziere baloons, but they sound interesting. One thing I've long wondered about would be the practicality of a balloon which had an inner vessel of H2O enclosed within a vessel of air which was heated above 100C. A lot of energy would be required on the ground to evaporate all the H2O in the inner vessel, but once that was accomplished the H2O would generate more than twice as much lift (assuming vessel temperature 375K and outside ambient ~290K). Ever heard of such a thing? $\endgroup$– supercatCommented Sep 8, 2014 at 18:22
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$\begingroup$ @supercat: Googling for "steam balloon" brings up a bunch of links suggesting that it can be done, and apparently has been done at least on a small scale, but it seems to belong in the category of nifty but impractical ideas. I suspect the big problem is the extremely steep sigmoid lift / heat curve near the condensation point, which seems likely to make such a balloon very hard to control safely. Besides, even if you could solve those safety and control issues, I can't think of a good reason to prefer such a steam balloon over a basic helium Rozière. $\endgroup$ Commented Sep 8, 2014 at 21:11
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$\begingroup$ @IlmariKaronen: Water vapor can be produced using a wide range of resources, while it's almost impossible to procure helium without a source of helium (alpha decay produces helium, but not in useful quantities). I'm not sure how the cost of energy using H2O would compare with the cost of helium. Do balloonists have pumps which can re-compress helium for storage when they're done with it? $\endgroup$– supercatCommented Sep 8, 2014 at 21:54
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1$\begingroup$ @supercat - the helium we use comes from alpha decay. We just get to take advantage (for the moment) of accumulation over geologic time scales. $\endgroup$ Commented Aug 26, 2015 at 13:05
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Actually, hydrogen is the only gas that is lighter than helium. However, it has a very big disadvantage: It is highly flammable. On the other hand, helium is almost completely inert - this is why it is very much safer to use the latter.
What might happen when you use hydrogen instead of helium was impressively proven by history when the "Hindenburg" zeppelin exploded on 6 May 1937. There is video footage, that can be seen on youtube.
In some of the comments it was mentioned, that hydrogen alone might not be the cause of the Hindenburg disaster, there were other contributing factors. However, using hydrogen remains dangerous, as this weather balloon experiment shows. In a more scientific setup the burning of a hydrogen balloon is compared to oxygen and a mixture of oxygen and hydrogen. Unfortunately a video of a helium filled balloon is not available, but it basically only ruptures and pops because of the different pressures on the in- and outside.
answered Sep 8, 2014 at 8:52
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$\begingroup$ I can't find a good source now, but I thought in addition to everything else mentioned, hydrogen also diffuses faster than helium meaning the balloon will deflate faster (though this is less of an issue for weather balloons than the 30 cent type party balloons) $\endgroup$– FoonCommented Sep 8, 2014 at 19:16
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2$\begingroup$ While hydrogen is indeed more flammable than helium, and is probably a contributing factor to the Hindenburg disaster, it is not completely certain that hydrogen was the primary cause. (See the article you linked to.) $\endgroup$ Commented Sep 8, 2014 at 20:10
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10$\begingroup$ @200_success Nitpick: Hydrogen is not "more flammable than Helium". Hydrogen is flammable. Helium is not. Helium is completely inert under all situations outside the core of a nuclear explosion or medium-sized stars. Helium (or any other group 18 element) will not oxidize (burn) by any process you'll find on this planet. $\endgroup$– davidCommented Sep 9, 2014 at 7:39
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3$\begingroup$ @david - I would say that that does actually qualify Hydrogen as more flammable. It's not hard to be more flammable than something that can't burn. If it burns at all, it's more flammable. ---- That said, it's still not a great way to phrase it. $\endgroup$– BobsonCommented Sep 9, 2014 at 14:56
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1$\begingroup$ There are likely multiple contributing factors to the Hindenburg disaster (f.e. the paint used had a composition very similar to thermite, a pyrotechnic compound). Hydrogen alone cannot be blamed, although it probably didn't reduce the damage. However, we are unlikely to get a conclusive answer about the accident any more (barring the existence of time travel!) $\endgroup$– Brian SCommented Sep 9, 2014 at 18:17
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Yes, hydrogen is lighter than helium but helium, on the other hand, is an inert gas (very less reactive). Also, hydrogen is highly flammable so that would make it unsafe to play with balloons.
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One counter-argument: Helium is essentially a "fossil gas", and there's a limited supply of easy-to-get helium (until we get practical fusion reactors running, at least). Hydrogen, on the other hand, is universally available in $\ce{H2O}$ and needs only a bit of electricity to break it out. Since helium has important industrial uses other than balloons, I expect that we will eventually find it becomes too expensive to throw away on toys.
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1$\begingroup$ leaks/diffuses out of the balloon more rapidly than helium does - exactly the opposite is true. Helium leaks more rapidly, because the hydrogen atoms exists only in paired molecules, so "H2" is an much bigger molecule as one "He" atom and therefore the "He" has much-much higher leakage... $\endgroup$– clt60Commented Sep 10, 2014 at 9:52
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$\begingroup$ Hmm... en.wikipedia.org/wiki/Helium says Helium diffuses 3 times after than air but only 65% of Hydrogen gas. Elsewhere, it's implied that effusion (leak) is primarily driven by mass not size; using 4 for mass of helium and 2 for hydrogen works out to the 65% (1/sqrt(2)) Anyone got a source for @jm666's counterargument? $\endgroup$– FoonCommented Sep 10, 2014 at 16:50
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$\begingroup$ @Foon aplying the simple Graham's law, the Hydrogen atoms should diffuse faster, but the molecule H2 is bigger than one Helium atom (not by molecular weight). (AFAIK) - going to search some relevant doccu - maybe I'm wrong... $\endgroup$– clt60Commented Sep 10, 2014 at 17:03
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$\begingroup$ @Foon just found: the atomic radius of the He is 28 (31) pm (by different sources). Now need find the molecular radius of the hydrogen molecule. $\endgroup$– clt60Commented Sep 10, 2014 at 17:19
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$\begingroup$ For what it's worth, according to Wikipedia, the van der waals radius of He =~ 2.11 A, and H2 is 1.91 A $\endgroup$– FoonCommented Sep 10, 2014 at 18:26
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With hydrogen, you are just one touch away from disaster. A hydrogen balloon goes anywhere near the birthday candles and you end up. Helium on the other hand is so inert that you can inhale it and all it would do is to make you sound like a chipmunk for a minute or so.
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4$\begingroup$ Not completely true. Filling balloons with hydrogen (or methane for that matter) and then making them explode with a flame was a common fun experiment performed in the last lectures before Christmas or carneval. The gist is that while it will burn, it won’t explode unless it’s a $2:1$ hydrogen : oxygen mixture. $\endgroup$– JanCommented Sep 29, 2016 at 15:02
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From my thinking helium is a stable gas and is a noble gas whereas hydrogen being the lightest gas but is flammable that's why helium is filled in weather balloons.
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$\begingroup$ Welcome to Chemistry.SE! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. This answer is really short and does not offer any new insight to the questions. Please consider expanding it. $\endgroup$– Martin - マーチン ♦Commented Jun 26, 2015 at 9:09
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1$\begingroup$ @Martin's right. This is sheer opinion, thus is considered better as a comment than answer. Please be more elaborate, cite resources for your claims etc. Otherwise, this may get deleted. $\endgroup$– M.A.R.Commented Jun 26, 2015 at 9:17