15.3.3—The root-mean-square speed cr.m.s. is given by c<>2

The root-mean-square speed c_rms is defined by c_rms=√⟨c²⟩ and for an ideal gas c_rms=√(3RT/M), where M is molar mass.

Use absolute temperature and molar mass in kg mol⁻¹; lighter gases move faster at the same temperature.

At fixed T, helium has a larger rms speed than oxygen because its molar mass is smaller.

c_rms is not the arithmetic mean speed and does not mean every molecule travels at that speed.