Which gas travels fastest at the same temperature?

• A. Neon
• B. Helium
• C. Argon
• D. Krypton

Answer: (B)

At a fixed temperature, lighter gas molecules move faster due to how kinetic energy is distributed among particles. All gases have, on average, the same translational kinetic energy at the same temperature, so (1/2) m v^2 is similar across gases. Since velocity squared is inversely related to mass, the lighter the molecule, the higher its speed must be to realize that same energy. The root-mean-square speed scales with the inverse square root of molar mass, so v_rms ∝ sqrt(1/M). Helium has the smallest molar mass (about 4 g/mol), much lighter than neon, argon, and krypton, so it travels fastest at the same temperature. The heavier gases move more slowly because their greater mass requires a smaller velocity to keep the same kinetic energy.