* Molecular weight: Chlorine gas (Cl₂) has a molecular weight of 70.9 g/mol, while argon (Ar) has an atomic weight of 39.9 g/mol. Chlorine molecules are heavier.

* Diffusion and Molecular Weight: Diffusion is the process of particles spreading out. Lighter particles move faster and therefore diffuse faster. Heavier particles move slower and diffuse slower.

In short: The heavier chlorine gas molecules will move more slowly than the lighter argon gas molecules, resulting in slower diffusion.