Neil Haddon asked:
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 weight. Itís about and eighth of the density of air. Hydrogen is about a sixteenth the density of air. So itíll float in air and will even float upwards. Youíd have thought that hydrogen would be a better gas as it would give slightly more lift than helium because itís lighter. This is true. The problem is 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 doesnít give you twice as much lift because the amount of lift you get is in its difference in density with [respect to] air. Itís actually only another sixteenth of the density of air. Itís a little bit better but not very much, so itís not worth the danger.
Helium is quite expensive, though, because itís a limited resource here on the planet. It's only created by radioactive decay on Earth. Atomic nucleuses emitting alpha particles that are actually helium nucleuses. They slow down and gain some electrons and turn into a helium atom. It tends to be found in oil wells where you get a gas-proof layer of rock above a load of rocks containing radioactive elements. They break down to helium. It floats up and gets 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.