What gas is the lightest?

Hydrogen is the lightest gas among the given options. It has the lowest molecular weight of all gases, with a molecular weight of approximately 2 grams per mole. This characteristic allows hydrogen to be much less dense than air, making it the lightest of all gases.

When considering the molecular weights of the other gases listed, oxygen has a molecular weight of about 32 grams per mole, helium comes in at around 4 grams per mole, and nitrogen is approximately 28 grams per mole. Hydrogen’s significantly lower molecular weight is what distinguishes it as the lightest gas in this context. Additionally, hydrogen's low density contributes to its applications in various industries, including its use in balloons because it provides buoyancy.