Neon is the fastest. Chlorine is the slowest.

Which gas diffuses the fastest?

The rate of effusion for a gas is inversely proportional to the square-root of its molecular mass (Graham's Law). The gas with the lowest molecular weight will effuse the fastest. The lightest, and therefore fastest, gas is helium.

What makes a gas diffuse slower?

Gaseous particles tend to undergo diffusion because they have kinetic energy. Diffusion is faster at higher temperatures because the gas molecules have greater kinetic energy. ... Graham's Law states that the effusion rate of a gas is inversely proportional to the square root of the mass of its particles.