Molecular Speeds: Helium vs. Xenon (Maxwell–Boltzmann)

Theory

At the same temperature all gases have the same mean kinetic energy, so lighter molecules move faster: $v_{rms}\propto \sqrt{T/m}$. Xenon (131 u) is about 5.7× slower than helium (4 u), since $\sqrt{131/4}\approx 5.7$. This is why helium escapes from balloons and planetary atmospheres.

What to observe

• Compare the histograms: the helium curve is wide and shifted right.
• Both boxes show the same temperature.

Try this

• Raise the xenon temperature until it matches helium’s speeds (≈ 33× T!).
• Try nitrogen and oxygen — the main gases of air.

Also available in: Velocidades moleculares: hélio × xenônio (Maxwell–Boltzmann) (Português) · Velocidades moleculares: helio vs. xenón (Maxwell–Boltzmann) (Español)