The average kinetic energy of the molecules in a gas sample depends only on the temperature, T. But given the same kinetic energies, a lighter molecule will move faster than a heavier molecule.
rms speed = √3RT/M where R=8.314 J/(mol • K) and M is molar mass in kilograms per mole. Note that a joule is the same as a kg • m2/s2.
What is the rms speed of N2 molecules at 287 K?
What is the rms speed of He atoms at 287 K?