Almost every gas that’s lighter than air in Earth’s atmosphere is either toxic, flammable, toxic and flammable, or useless unless heated. The exception is helium.
Wikipedia’s page for lifting gas spells out the problem. All prospects for lighter-than-air flight stand in the long shadow of the Hindenburg disaster, but it’s worth noting that the people who filled the Hindenburg with hydrogen weren’t stupid. The U.S. had passed the Helium Control Act of 1927, which forbid export of helium. The Hindenburg was designed to use helium, and forced to make do with hydrogen.
You may be wondering why the U.S. had helium and Germany didn’t, and the answer is startlingly weird. As detailed in this morning’s Day Thread, stars fuse hydrogen into helium, but that’s not where the helium on Earth comes from. Like hydrogen, atmospheric helium rises to the top of the atmosphere and is blown away by the solar wind. No, the helium we use comes from underground.
Fusion is when small atoms stick together to make bigger atoms; fission is when large, unstable atoms break down into smaller atoms. There are many different fission and fusion paths, and so there’s more than one way to make helium. Deep in the Earth’s crust, fission gives birth to alpha particles, which are nothing more than helium nuclei spit out by larger atoms that don’t have their lives together. Commercial helium production is the separation of this subterranean helium from natural gas. Then the helium goes into balloons or MRIs or specialized atmospheres for welding or growing crystals. Then it escapes, floats to the top of the atmosphere, and is blown away. Helium is non-renewable, which is going to be a problem at some point.
I’d also mention chemical properties, but there’s not much to mention. Helium is the first of the noble gasses, which strongly resist becoming part of chemical reactions.