The week of Feb. 29, 2016, PBS aired a documentary on sending humans halfway to space in the late 1950’s using balloons. They filled different balloons with different gases— some with H2 and some with He. Which balloon should go higher? Why?

Hydrogen

Explanation:

Balloons are filled with light gases in order to make them float. Helium and Hydrogen are both light gases.

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.

Hydrogen however is twice as heavy as helium, but that doesn't mean the amount of lift off of the balloon is twice as much. The amount of lift is determined by the difference in density with respect to air.

Hydrogen is calculated to be less dense than helium, that explains why hydrogen filled balloons would go up higher.

Helium filled balloons are used because of the safety implications. The problem is that hydrogen is explosive may have some health and safety implications! Helium is much more safe to use.