Helium

Helium. On the face of it, rather a boring element. Doesn't really react chemically with anything. Most people's only contact with helium is in the form of helium balloons. Actually, though, helium is a very valuable resource. You might think that because it's very light, and all elements apart from hydrogen are the result of nuclear reactions, that it is common. And you'd be right - it makes up about 10-20% of all mass in the universe. The rest is hydrogen - all the other elements combined are insignificant. Down here on earth things are different, though. Here we have gravity to worry about. When helium reaches the upper atmosphere, some of it escapes into space. The reason for this is that at any given temperature, a volume of gas will contain some particles that are traveling faster and some that are traveling slower. The average of all these velocities gives the temperature. In the upper atmosphere some helium particles travel so fast that they escape the earth's gravity well. Escape velocity: Ve = sqrt(2Gm/r) where G is the universal gravitational constant, m is the mass of the body and r is the distance from the center of mass. Escape velocity from the earth's surface is about 11.2 km/s, which is of course VERY fast, faster than anything. It's about three hundred times the speed of sound. In the uppermost reaches of the atmosphere, at an altitude of 10,000 km the escape velocity is only 8.9 km/s. Now, of course, heavier gases will have lower RMS velocities because RMS kinetic energy is the same for all gases at a given temperature. Radon, one of the heaviest gases, is about fifty times heavier than helium. Kinetic energy: Ek = 1/2mv^2 where m is the mass of the body and v is its velocity. So radon's RMS velocity is just 13% that of helium. Heavy gases are not buoyant enough to reach the upper atmosphere though. Helium and hydrogen are and a lot of both of these gases is lost each year. Luckily we have plenty of hydrogen, locked away in water and other compounds. But helium doesn't form any compounds, because its electrons are very tightly bound, so it just migrates to the upper atmosphere and escapes. Jupiter doesn't have this problem so severely - it has such a powerful gravitational field that not even hydrogen can escape in any great amount. The same is true of Saturn and the other gas planets.
So our helium is escaping, slowly. So what. Well, there are a few natural processes that produce more helium, chief among which is radioactive decay. When Uranium decays eventually to lead it releases a few atoms of 4He on the way there, and these get trapped underground in oil wells and natural gas, which is where we get most of our helium from. Some 3He is formed by interactions in the upper atmosphere as well - when cosmic radiation starts frying nitrogen that's up there. These are the only sources of helium on earth, so we would be wise not to be so cavalier about the way we use it - balloons for instance.
So what is helium good for? Well, if you want to go SCUBA diving really far down, you're going to have to deal with the problem of the bends, eventually. At depth, the air you're breathing has to be higher pressure, to match the pressure of the water, and at high pressure the solubility of nitrogen in your blood increases dramatically. When you return to the surface too quickly the nitrogen becomes less soluble, and can't be breathed out fast enough, so it forms bubbles in your blood, same way a carbonated drink forms bubbles when it's opened. Helium has such a high RMS velocity that it doesn't dissolve in your blood very much at all, so if you breathe oxygen mixed with helium, you can go down to much greater depths and pressures without worrying too much about decompression.
Helium is also very useful for low temperature physics. Helium has no nett dipole moment, so helium atoms don't attract one another very well at all. It's also very light which means that it's fast, and quantum mechanical juggling due to uncertainty is very big. All this means that helium has to be at a very low temperature before it will liquefy. Helium doesn't freeze at all. Ever. So you can use it as a refrigerant gas for making things very cold because it's still a gas at 5K. It also behaves very strangely at these temperatures. It becomes a superfluid at around 2K - it's a sharp changeover, much like a change from liquid to solid. This means that it stops behaving like a regular fluid and starts behaving like a quantum mechanical fluid. This means, for one thing, that it can only have integer values of turbulence and rotation. Practically, this means that you can spin a bucket of liquid helium and the fluid will stay still until you spin the bucket fast enough for the first rotational quantum value. So it has stepwise rotation. It also flows with zero friction though fine tubes because it cannot become turbulent. It can only allow eddies of a certain size and angular momentum, and below this size no eddies can form, so no energy can be dissipated. An experiment has been conducted where a hollow torus was filled with fine powder and then liquid helium. The helium was set flowing around the ring, and it never stops. Water molecules would bash into the fine powder and stop flowing very quickly, but helium can't become turbulent at that scale so it just keeps on flowing.
It's interesting stuff, helium. Think about that next time you let a balloon float up into the sky.