Why do we use helium and not hydrogen balloons?
Why is it helium in balloons and not hydrogen?
In order to get a floating balloon you want a gas which is as light as possible. Helium is quite a lot lighter than air: it's about an eighth of the density of air. Hydrogen is about a sixteenth the density of air, so it'll float in air and will in fact float upwards.
You'd have thought that hydrogen would be a better gas as it would give slightly more lift than helium because it's lighter. This is true. The problem is that hydrogen is explosive, and if you have children running around with balloons that could catch fire and blow up in their faces, it may have some health and safety implications!
The other thing is that, although hydrogen is half as heavy as helium, it doesn't give you twice as much lift because the amount of lift you get is determined by the difference in density with respect to air. So it's actually only another sixteenth of the density of air, meaning it's a little bit better but not very much, so it's not worth the danger.
Helium is quite expensive, though, because it's a limited resource here on the planet. It's only created by radioactive decay on Earth: atomic nuclei emitting alpha particles. These are actually helium nuclei. They slow down and gain some electrons and turn into a helium atom.
Helium tends to be found in oil wells where you get a gas-proof layer of rock above a load of rocks containing radioactive elements. These break down to helium. This floats up and becomes trapped, often at the top of an oil well . The amount of helium that we can access cheaply is very limited because not all our oil wells have it.