Gases lighter than air include which of the following gas types?

• A. Helium
• B. Ammonia
• C. Neon
• D. Hydrogen

Answer: (D)

Gas buoyancy comes from density: a gas will rise if its density is less than that of the surrounding air. Hydrogen has the smallest molar mass (H2 is about 2 g/mol) and the lowest density of the listed gases (around 0.089 g/L at STP), so it is by far the lightest. That makes it the clearest example of a gas that is lighter than air and capable of rising, which is why it’s the best answer. Helium is lighter than air too, but not as light as hydrogen; ammonia and neon are lighter than air as well but heavier than hydrogen, so hydrogen stands out as the lightest option here.