# Why do hydrogen and helium float upward? **URL:** \ **Category:** Factual Questions\ **Created:** [January 25, 2012, 3:11pm UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601 "2012-01-25T15:11:21Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![zoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zoid/32/336_2.png) [@zoid](https://boards.straightdope.com/u/zoid)\ **Post date:** [January 26, 2012, 3:00am UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/21 "2012-01-26T03:00:10Z")
> [@YamatoTwinkie](#): > > Pressure differentials. > > For something like a pool of water, the further down you go, the more pressure there is. (The pressure is equal to the weight of all the water directly above it) If you built a hollow cube, filled it with water, and submerged it in the pool, the pressure on the top surface of the cube is going to be lower than the pressure at the bottom surface of the cube (since the bottom surface is further down where there is more pressure). This pressure differential causes an upward force on the cube. However the water cube stays exactly where it is because the weight of the water inside the cube is also pushing down _exactly_ the same amount as the pressure differential is pushing upwards. > > But say you took the same hollow cube and filled it with air, then submerged it in the pool. The pressure differential between the top surface and the bottom surface of the cube is going to be the same as the previous example, which again pushes up on the cube. But this time, there is nothing inside the cube weighing it down to counteract that force. So the air cube rises. > > The same thing happens in the atmosphere with a balloon of helium. There’s a (tiny) pressure differential between the air at the bottom of the balloon and the air at the top of the balloon. Since the helium isn’t heavy enough to counteract the pressure differential, the balloon rises. I like this one the best. ---
**Author:** ![JWT\_Kottekoe](https://avatars.discourse-cdn.com/v4/letter/j/3ec8ea/32.png) [@JWT\_Kottekoe](https://boards.straightdope.com/u/JWT_Kottekoe)\ **Post date:** [January 26, 2012, 3:06am UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/22 "2012-01-26T03:06:22Z")
For most purposes, we can consider the atmosphere to be composed of “ideal gases,” which means that we can consider all the molecules to be non-interacting, except to the extent that the collisions thermalize the molecules. That means that the direction of motion is rapidly randomized and that the speed assumes a bell-shaped curve. The square of the average velocity is proportional to the temperature (in degrees above absolute zero) and inversely proportional to the mass of the molecule. Another way to say this is that, on average, each molecule has an energy proportional to the temperature. Thus, since helium weighs 4 daltons and nitrogen 28, the speed of the helium molecules is the square root of 7 times as fast (~2.5x). The molecules of one gas diffuse through all the others, more or less oblivious to the presence of the others, so you can treat each one separately. The nitrogen atoms don’t push the helium atoms out of the way, or vice versa. The average height of the molecules above the ground is the height at which the gravitational potential energy of the molecule is equal to the thermal energy, so again that height is proportional to the absolute temperature and inversely proportional to mass, so helium goes, on average, about 2.5x higher. Just like the nitrogen, the concentration of helium is highest at sea level and tends to go down exponentially with height. The height at which the nitrogen is half as plentiful is about 5 miles, for helium it would be about 13 miles. All of this is an approximation of course, but for many purposes it is a very good one and gives the correct physical intuition about what is going on and WHY. There is no layering, as pointed out above, and it does not have to do with atoms pushing each other out of the way. ---
**Author:** ![DHMO](https://avatars.discourse-cdn.com/v4/letter/d/ecc23a/32.png) [@DHMO](https://boards.straightdope.com/u/DHMO)\ **Post date:** [January 26, 2012, 3:49am UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/23 "2012-01-26T03:49:21Z")
> [@Trinopus](#): > > Isn’t the meta-question, “why are Helium atoms ‘fat?’” Why does a given volume of Helium, at standard temperature and pressure, have so few atoms, so that it masses so very much less than, say, the same volume of Lithium atoms (even allowing for Lithium atoms being heavier in atomic number.) i.e., why does Helium resist compression so much more than Nitrogen or any of the solids? > > I _think_ it has to do with the outer electron shell being completely full, as in all of the noble gases… > > Edited to Add: I don’t know what the hell I’m talking about, but I’m trying to ask intelligent questions… The basic answer to this question is that most elemental gasses are diatomic molecules, i.e., O[sub]2[/sub], H[sub]2[/sub], N[sub]2[/sub] and so forth. This has to do with the covalent bond being a lower energy state for these atoms than free individual atoms would have. You are correct in your intuition about the filled electron shell, as Helium (as well as the other noble gasses) has very little inclination to bond with anything, and exists as a monatomic gas. There are roughly the same number of _particles_ in a constant volume of gas at STP (Standard Temperature and Pressure) regardless of the size or mass of the individual particles. For diatomic gasses, each particle consists of two atoms, so Oxygen, atomic number 8, atomic mass 16, has a molecular weight of 32, whereas Helium, atomic number 2, atomic mass 4, does not form a molecule and has a “molecular” weight of 4. Hydrogen (Protium, to be exact), with atomic number 1 and atomic mass 1 forms a diatomic molecule with molecular weight of 2, and a Hydrogen-filled balloon would float in a Helium-filled container. ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [January 26, 2012, 4:01am UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/24 "2012-01-26T04:01:02Z")
Deuterium of course has a very small but non-negligible effect on the molecular weight of hydrogen – three atoms in 10,000 are deuterium, itself having an atomic weight of two and a molecular weight when combined with protium of three. (D[sub]2[/sub], along with tritium and Helium-3, occur so rarely as to be truly negligible for our purposes.) So if two-protium hydrogen molecules have a molecular weight of 2 even (ot quite true but a reasonable approximation), actual hydrogen has a weight of 2.0003 – marginally more than half the weight of a helium atom. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [January 26, 2012, 11:30am UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/25 "2012-01-26T11:30:58Z")
> [@notsoheavyd3](#): > > Oh you’d see other effects besides that if it were true. For example you’d expect to see wine, beer, or other alcoholic liquids stratify into a layer of water on top, then a layer of ethanol(which is heavier) and then a layer of even heavier stuff. (Because the liquids in both beer and wine are mixtures just like air is a mixture of gases.) I’m curious as to \*why \*this is the case at the molecular/atomic level, but different in bulk. Is it just because the buoyancy of an individual atom of substance X is vastly overwhelmed by other force inputs such as Brownian motion, intermolecular forces, thermal convection, turbulence, etc? Or is it that buoyancy, as a phenomenon, just doesn’t exist below a certain scale? (in which case, why and how?) ---
**Author:** ![YamatoTwinkie](https://avatars.discourse-cdn.com/v4/letter/y/cc9497/32.png) [@YamatoTwinkie](https://boards.straightdope.com/u/YamatoTwinkie)\ **Post date:** [January 26, 2012, 1:41pm UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/26 "2012-01-26T13:41:46Z")
> [@Mangetout](#): > > Is it just because the buoyancy of an individual atom of substance X is vastly overwhelmed by other force inputs such as Brownian motion, intermolecular forces, thermal convection, turbulence, etc? This. How fast does a helium balloon go once released? Maybe 1 meter/second? Then try calculating the [Root-mean-square speed](http://en.wikipedia.org/wiki/Root-mean-square_speed)of helium. Even at moderate temperatures, this can be on the order of 1000 m/s. Once confined to a balloon, however, RMS speed turns into “average speed”, which is zero, leaving you with only buoyancy. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [January 26, 2012, 2:47pm UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/27 "2012-01-26T14:47:02Z")
> [@YamatoTwinkie](#): > > This. How fast does a helium balloon go once released? Maybe 1 meter/second? > > Then try calculating the [Root-mean-square speed](http://en.wikipedia.org/wiki/Root-mean-square_speed)of helium. Even at moderate temperatures, this can be on the order of 1000 m/s. > > Once confined to a balloon, however, RMS speed turns into “average speed”, which is zero, leaving you with only buoyancy. Well, OK, but if the molecules are zipping about very fast in random directions, then shouldn’t all those RMS speeds average out to zero over time, also leaving only buoyancy? ---
**Author:** ![YamatoTwinkie](https://avatars.discourse-cdn.com/v4/letter/y/cc9497/32.png) [@YamatoTwinkie](https://boards.straightdope.com/u/YamatoTwinkie)\ **Post date:** [January 26, 2012, 3:20pm UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/28 "2012-01-26T15:20:39Z")
> [@Mangetout](#): > > Well, OK, but if the molecules are zipping about very fast in random directions, then shouldn’t all those RMS speeds average out to zero over time, also leaving only buoyancy? Say you open a casino that has thousands of tables of 24/7 high-stakes poker. You get $1 for each hand that everyone plays. From your perspective, you’re making constant income. From the individual player’s perpective, that $1/hand is noise-level. They can go from rich-to-poor and back again in just a few hands. One lucky streak for an individual can pay the house fees for a thousand hands of poker. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [January 26, 2012, 4:10pm UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/29 "2012-01-26T16:10:37Z")
> [@YamatoTwinkie](#): > > Say you open a casino that has thousands of tables of 24/7 high-stakes poker. You get $1 for each hand that everyone plays. > > From your perspective, you’re making constant income. From the individual player’s perpective, that $1/hand is noise-level. They can go from rich-to-poor and back again in just a few hands. One lucky streak for an individual can pay the house fees for a thousand hands of poker. But in the big picture, the house always wins, and the punter therefore - on average - walks away poorer. So why, in the big picture, doesn’t the atmosphere generally stratify and the helium - on average - float to the top? ETA: I mean, I accept that it really doesn’t, but I just don’t see why yet. ---
**Author:** ![grimpixie](https://avatars.discourse-cdn.com/v4/letter/g/ecb155/32.png) [@grimpixie](https://boards.straightdope.com/u/grimpixie)\ **Post date:** [January 26, 2012, 4:40pm UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/30 "2012-01-26T16:40:40Z")
The atmosphere keeps getting mixed? ---
**Author:** ![california\_jobcase](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@california\_jobcase](https://boards.straightdope.com/u/california_jobcase)\ **Post date:** [January 26, 2012, 5:06pm UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/31 "2012-01-26T17:06:59Z")
Helium on Earth is a product of alpha radiation. The atmosphere doesn’t contain significant amounts of it because any helium that finds its way into the atmosphere eventually escapes into space, since the earth’s gravity isn’t strong enough to keep it here. The same goes for hydrogen gas. Jupiter and Saturn have stronger gravitational fields, so their atmospheres have a good bit of helium in them. Any mixture of room-temperature gases trapped in a jar on Earth will not stratify- they’ll stay mixed. Open the lid, and hydrogen and helium will mix and rise in the atmosphere faster than the heavier fractions in the jar. Yes, the atmosphere is constantly mixing, but eventually, helium atoms will end up high enough to escape into space. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [January 26, 2012, 5:37pm UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/32 "2012-01-26T17:37:09Z")
> [@Mangetout](#): > > I’m curious as to \*why \*this is the case at the molecular/atomic level, but different in bulk. I think it’s because buoyancy acts on the _boundary_ of the bulk. One way to think about buoyancy is as a pressure difference between the top and bottom boundary. The top surface of the balloon is at a higher elevation than the bottom, so the atmospheric pressure is slightly lower at the top. Which means the air pressure pushing down on the top surface is smaller than the pressure pushing up against the bottom surface. If the balloon were filled with air, this difference in pressure is just enough to counteract the weight of the air. But hydrogen and helium are not heavy enough to counteract the buoyancy, so the whole balloon goes up. ---
**Author:** ![iamthewalrus\_3](https://avatars.discourse-cdn.com/v4/letter/i/258eb7/32.png) [@iamthewalrus\_3](https://boards.straightdope.com/u/iamthewalrus_3)\ **Post date:** [January 26, 2012, 5:43pm UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/33 "2012-01-26T17:43:54Z")
> [@Mangetout](#): > > But in the big picture, the house always wins, and the punter therefore - on average - walks away poorer. So why, in the big picture, doesn’t the atmosphere generally stratify and the helium - on average - float to the top? I believe the answer is that the Earth’s atmosphere is a dynamic system. The earth is rotating on its axis, and orbiting the sun, and there are tidal forces, and so on. There’s a bunch of energy inputs into the atmosphere that cause mixing. If the Earth were plucked out of this universe and put into another one with no external sources of energy to cause mixing (and ceasing its rotation), then that’s exactly what you would see. The atmosphere would stratify with each gas forming a layer. Although it might all freeze before that happened, because black body radiation would be shedding heat energy from the Earth, and there’d be nothing to replace it. ---
**Author:** ![blood63](https://avatars.discourse-cdn.com/v4/letter/b/97f17d/32.png) [@blood63](https://boards.straightdope.com/u/blood63)\ **Post date:** [January 26, 2012, 6:23pm UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/34 "2012-01-26T18:23:11Z")
[quote=“california\_jobcase, post:31, topic:610601”] Helium on Earth is a product of alpha radiation. The atmosphere doesn’t contain significant amounts of it because any helium that finds its way into the atmosphere eventually escapes into space, since the earth’s gravity isn’t strong enough to keep it here. The same goes for hydrogen gas. Jupiter and Saturn have stronger gravitational fields, so their atmospheres have a good bit of helium in them. QUOTE] Explain what you wrote here to me. From what I have learned so far, a helium atom is moving faster than other atoms to keep the same temperature so it eventually zings its way upwards. I can kind of see that. But according to his quote, some other mysterious force, stronger than gravity causes the helium to continue on its journey into the vacuum of space. What am I missing? Should the outer atmosphere not be ringed with a layer of helium? ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [January 26, 2012, 6:29pm UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/35 "2012-01-26T18:29:32Z")
> [@blood63](#): > > Explain what you wrote here to me. > > From what I have learned so far, a helium atom is moving faster than other atoms to keep the same temperature so it eventually zings its way upwards. I can kind of see that. > But according to his quote, some other mysterious force, stronger than gravity causes the helium to continue on its journey into the vacuum of space. > What am I missing? > Should the outer atmosphere not be ringed with a layer of helium? Not everything that goes up comes down. If you’re going fast enough, (past escape velocity) your inertia keeps you going, as the force of gravity decreases when you leave the earth behind. I don’t know if individual helium atoms have escape velocity as they leave the layer of the outer atmosphere where they’d be likely to get additional energy from bumping into heavier atoms, but it sounds quite plausible. ---
**Author:** ![california\_jobcase](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@california\_jobcase](https://boards.straightdope.com/u/california_jobcase)\ **Post date:** [January 26, 2012, 6:49pm UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/36 "2012-01-26T18:49:26Z")
Blood63, yep, there’s helium out there- > **[Exosphere](https://en.wikipedia.org/wiki/Exosphere)** > > The exosphere (Ancient Greek: ἔξω éxō "outside, external, beyond", Ancient Greek: σφαῖρα sphaĩra "sphere") is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is so low that the molecules are essentially collision-less. In the case of bodies with substantial atmospheres, such as Earth's atmosphere, the exosphere is the uppermost layer, where the atmosphere thins out and merges with outer space ---
**Author:** ![blood63](https://avatars.discourse-cdn.com/v4/letter/b/97f17d/32.png) [@blood63](https://boards.straightdope.com/u/blood63)\ **Post date:** [January 26, 2012, 6:53pm UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/37 "2012-01-26T18:53:19Z")
> [@chrisk](#): > > Not everything that goes up comes down. If you’re going fast enough, (past escape velocity) your inertia keeps you going, as the force of gravity decreases when you leave the earth behind. > > I don’t know if individual helium atoms have escape velocity as they leave the layer of the outer atmosphere where they’d be likely to get additional energy from bumping into heavier atoms, but it sounds quite plausible. I will await experts on this, but it don’t see how the decrease in the force of gravity at that altitude could be a factor. At 100 km up, the force of gravity is essentially the same at the surface. ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [January 26, 2012, 7:08pm UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/38 "2012-01-26T19:08:34Z")
> [@blood63](#): > > I will await experts on this, but it don’t see how the decrease in the force of gravity at that altitude could be a factor. > At 100 km up, the force of gravity is essentially the same at the surface. The decrease in the force of gravity at 100km up is not the key factor. You need enough speed to keep climbing against gravity until the force of gravity does decrease until it’s negligible. This is a lot of speed/momentum/initial kinetic energy, but it’s possible. You could theoretically build a catapult on the earth’s surface strong enough to fling a rock out past the edges of the solar system. (But if you tried to use it for a manned space capsule, the passenger’s bodies would get smushed by the initial acceleration.) ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [January 26, 2012, 8:07pm UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/39 "2012-01-26T20:07:01Z")
> [@blood63](#): > > I don’t know if individual helium atoms have escape velocity as they leave the layer of the outer atmosphere where they’d be likely to get additional energy from bumping into heavier atoms, but it sounds quite plausible. They perhaps don’t need escape velocity - just enough to get them to the point where the solar wind whisks them away. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [January 30, 2012, 2:14am UTC](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601/40 "2012-01-30T02:14:31Z")
Will stratification occur In outer-space (no atmospheric mass) orbit? Yes, I think, on a constant normal to the tangent… Will the balloon rise in a micro- vacuum, air atmosphere (eg, the ISS)? [Previous page](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601.md?page=1) [Next page](https://boards.straightdope.com/t/why-do-hydrogen-and-helium-float-upward/610601.md?page=3)