Hydrogen was once used to fill airships. Quite dangerous due to how flammable it is.
That'd be hydrogen.
Helium
Hydrogen and helium are two elements that are kinda-sorta interchangeable in airships.
The only answer that comes to mind is "Airships". Hydrogen, the lightest element, was used in airships, where excess weight was undesirable. However, accidents like the Hindenburg disaster showed that Hydrogen gas was too dangerous(it is extremely flammable) to be used. Since then, Helium has been used instead. Helium is also a very light element, and is completely safe, as it is inert.
because hydrogen used to be in those airships and since hydrogen is flammable it was very dangerous
Hydrogen was the gas that was once used to filled airships. It is extremely plentiful and easy to produce. However, it is also extremely flammable and caused a few accidents. Helium is now used.
Hydrogen (explosive), Helium (non-explosive).
Hydrogen used to be used in airships.
gas is still used in airships to provide lift.
Hydrogen
Modern airships use Helium for lift which is non-combustible whereas in the past airships such as the Hindenburg used Hydrogen for lift which is highly combustible. In addition, modern airships take advantage of new alloys and polymers which give them a much stronger airframe.
As a radioactive and unstable element roentgenium can be dangerous.