# High Viscosity of Noble Gases

Why are the viscosities of noble gases higher than almost all other gases? This seems counter-intuitive since they have much weaker inter-molecular interactions and lower boiling points.

Gas Viscosity Table: http://www.engineeringtoolbox.com/gases-absolute-dynamic-viscosity-d_1888.html

• I wonder if this is at all related to the unintuitive fact that gasses increase in viscosity as temperature is increased, unlike the majority of liquids. Noble gasses and higher temperatures both provide situations which behave closer to an ideal gas. Jan 9, 2016 at 9:45
• It is very related. Higher gas temperature means higher particle velocity. Higher velocity means more collisions. More collisions means higher viscosity. Jul 18, 2016 at 20:49
• In the case of temperature particle velocity is increased by higher temperature as opposed to lighter particles (He, Ne, Ar.) Jul 18, 2016 at 20:55

## 1 Answer

Viscosity in gasses is generally different than liquids. Like liquids exhibiting fluidic properties, the viscosity is related to internal friction; however, the friction mechanism is different than inter-molecular interactions experienced by closely packed molecules. The molecules in a gas are far apart resulting in the influence of an entirely different friction mechanism.

The friction mechanism dominating gas viscosity is related to the number of collisions between gas particles. More collisions means more internal friction. In gasses, two dominant factors affect the number of collisions: (1) the mean distance a gas particle has to travel before it collides with another particle and (2) the average velocity of the particle within the gas.

Some noble gasses (He, Ne, Ar) are monatomic particles with a lighter mass than the diatomic elemental gasses. The light mass results in a much higher velocity particles at a given temperature (consider $E=\frac{3}{2}kT=\frac{1}{2}m \bar{v^2}$) resulting in more collisions. Xenon has a high molecular diameter, so the shear increase in size of the molecule increases the probability of a collision, thus reducing the mean distance it travels before a collision.