Lighter-than-air

Lighter than air refers to materials (usually gases) that are buoyant in air because they have average densities lower than that of air. Dry air has a density of about 1.29 g/L (gram per liter) at standard conditions for temperature and pressure (STP), and an average molecular mass of 28.97 g/mol.

Some of these gases are used as lifting gases in lighter-than-air aircraft, which include free balloons, moored balloons, and airships, to make the whole craft, on average, lighter than air. (Heavier-than-air aircraft include airplanes, gliders and helicopters).

Hot air is frequently used in recreational ballooning. Hot air is lighter than air at ambient temperature, to a degree which can be approximated using the ideal gas law.

Neon (density 0.900 g/L at STP, average atomic mass 20.17g/mol) is lighter than air and will lift a balloon. However, it is relatively rare on Earth, expensive, and is among the heavier of the lifting gases.

Water vapor (density 0.804 g/L at STP, average molecular mass 18.015 g/mol) is lighter than air, and has successfully been used as a lifting gas. It is generally impractical due to high boiling point and condensation.

Ammonia (density 0.769 g/L at STP, average molecular mass 17.03 g/mol) has sometimes been used to fill weather balloons. Due to its relatively high boiling point (compared to helium and hydrogen), ammonia could potentially be refrigerated and liquified aboard an airship to reduce lift and add ballast (and returned to a gas to add lift and reduce ballast).

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Wikipedia