# What is the most viscous gas

A couple interesting points I dredged up:

In this web page a formula for viscosity in a gas is derived:

Visc = 5(MRT/p)1/2/(16NAs2)

where M is molecular weight, R is the gas constant, T is temperature, NA is Avagadro's number, and s is the collision diameter of the molecule. As they point out, this means that viscosity is dependent on temperature, but not on pressure.

More importantly, the highest-viscosity gas should be the one that combines high molecular weight and low collision diameter (and note that viscosity is much more sensitive to the diameter than the weight!).

Also, from here: perfluorobutane viscosity (25C) = 0.01454 cP.

From my CRC, some viscosities:
Air (18C) 0.01827 cP
Oxygen (19C) 0.02018 cP
Helium (20C) 0.01941 cP
Neon (20C) 0.03111 cP
Argon (20C) 0.02217 cP
Krypton (15C) 0.0246 cP
Xenon (20C) 0.02260 cP

I hypothesize that organic compounds like perfluorobutane, although heavier, have a large enough collision diameter that the actual viscosity is pretty low. Furthermore, I suggest that neon might be about as viscous a gas as you can get. Note, though, that the viscosity of neon is less than twice that of air, say, so it's not like there's a huge jump in viscosity. 16