What is the second lightest gas?

Answer 1

The second lightest gas is helium. The lightest gas is hydrogen which is also the lightest element. They are very close on the Periodic Table.

Answer 2

If you count only chemical substances in the usual sense, the third lightest gas, after hydrogen and helium, is methane (CH4), the main component of natural gas, at a density of about 0.67 grams per litre - which is four times as much as the second lightest gas helium. It can still be used as a lifting gas for balloons and airships, but it has only about half the lifting power of hydrogen or helium, so you need twice as much gas to lift the same weight. Like hydrogen, it is highly flammable, but unlike hydrogen and helium, it doesn't escape all that quickly through the skin of a balloon.

There is a gas that is theoretically lighter than methane which however doesn't exist in pure form. The gas is called monoborane (BH3) but it can only exist as a mixture with diborane (B2H6) or other boranes; pure monoborane, if you could make it, would automatically becomes mostly diborane after a very short time. The borane mixtures are all heavier than methane.

If you count isotopes, the third lightest gas is helium-4 ("normal" helium), after hydrogen-1 ("normal" hydrogen) and the extremely rare helium-3. Hydrogen-2 (also called deuterium) comes in for a very close fourth place, as it is just a tiny bit heavier than helium-4.

Answer 3

The second lightest gas is helium. The lightest gas is hydrogen which is also the lightest element. They are very close on the Periodic Table.