The rate of diffusion of gases is inversely proportional to the square root of their molar masses. In other words, the lighter the molecule, the faster it will diffuse.

The molar mass of chlorine (Cl₂) is 70.9 g/mol, while the molar mass of argon (Ar) is 39.95 g/mol.

Therefore, argon will diffuse faster than chlorine because it has a lower molar mass.