How do the properties of gases make a helium balloon behave the way it does?
It's fluid properties that make the balloon float: fluids are either liquids or gases.
A helium balloon displaces a volume of air, that here, clearly, has a greater mass than the mass of balloon, and the mass of the helium gas it contains. Helium has a molecular mass approx. 7 times less than that of air.
Because the helium balloon displaces a volume of air that has a greater mass than the mass of the filled balloon, and the basket, and the punter riding in the basket, there is a net buoyant force on the balloon that causes the balloon to rise. This is Archimedes' principle, and has been known since the 4th century, BC.
Hydrogen gas has less intrinsic mass than helium, and this would be an even better lift gas, however, helium has one important chemical property that dihydrogen does not have; what is it? If you don't see this immediately, then consider the fate of the