The mass of a deuterium (diatomic) molecule is roughly the same as that of an ordinary helium (monatomic) atom. If deuterium gas is at the same temperature as helium gas, the rms speeds of the deuterium molecules is

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The rms speed of gas molecules is inversely proportional to the square root of their mass (v_rms ∝ 1/√m). Since the mass of a deuterium molecule is roughly the same as that of a helium atom, their rms speeds would be approximately the same at the same temperature.

The Mass of a Deuterium (Diatomic) Molecule Is Roughly the Same