## Conceptual Physics (12th Edition)

Having the same temperature means they have the same average translational kinetic energy per molecule. $KE = \frac{1}{2} mv^{2}$. We may check the atomic masses in the periodic table (page 216) and verify that the mass m of an argon atom is much less than that of a krypton atom, so the speed v must be greater to have the same average kinetic energy.