## Conceptual Physics (12th Edition)

On average, hydrogen molecules move faster than oxygen and nitrogen molecules at the same temperature. Having the same temperature means the gases have the same average translational kinetic energy per molecule. $KE = \frac{1}{2} mv^{2}$. The mass m of a hydrogen molecule is much less than that of an oxygen or a nitrogen molecule, so the speed v must be greater to have the same average kinetic energy.