Here's why:

* Molecular weight: Hydrogen gas (H2) has a molecular weight of 2 g/mol. Air is a mixture of gases, primarily nitrogen (N2) and oxygen (O2), with an average molecular weight of about 29 g/mol. This means hydrogen molecules are much lighter than the average air molecule.

* Density: Density is mass per unit volume. Because hydrogen molecules are much lighter, a given volume of hydrogen gas will have a much lower mass than the same volume of air.

This is why hydrogen balloons float! The lighter hydrogen gas displaces a volume of air that weighs more, creating buoyancy.