Why is mercury's surface tension so high, when its viscosity is low?

At a basic level, both surface tension and viscosity are the result of forces between particles. Usually we'd talk about intermolecular forces, because liquids are usually molecules, but obviously that doesn't work for mercury, which isn't made of molecules at all. Stronger forces between particles give higher surface tension, and usually also higher viscosity. So water's fairly high surface tension is usually credited to hydrogen bonding, and mercury's exceptionally high surface tension is put down to metallic bonding; while the increasing viscosity of aliphatic hydrocarbons with more carbons is said to be due to increasing London dispersion forces.


It's easy to see that the two are poorly correlated, though. Ethanol has almost identical surface tension to acetone, for example, but it's more than three times as viscous. Most strikingly, mercury has really extraordinarily high surface tension, but is no more viscous than cold water. 


Is it *completely* wrong to expect at least some kind of loose correlation between surface tension and viscosity? Is viscosity just a far more complex phenomenon, a proper explanation of which lies way beyond the hand-wavy school science that chemistry teachers use to give a vague idea about things like trends in the alkanes? Would it be better to emphasise entanglement of long molecules when trying to explain the viscosity of longer alkanes, rather than just stronger forces?


Is mercury's low viscosity mainly down to its small particle size, or is there much more to it than that?




I think the complex phenomenon here is the nature of the metallic bond. It is commonly said there is a "sea of electrons" that are delocalized across the entire metal and that the metal ions just bash around in the sea against each other. So when they're in the metal bulk phase they feel about the same **strong force** on all sides (thus as Ivan said, the force is undirected). Therefore in the liquid the atoms can slip past each other OK. The problem with the vapor phase is that it's no longer metallic. An ideal gas (an excellent approximation for mercury or most other vapors at STP) has **no forces** between molecules, by definition. So you've got to pull the metal atom completely out of the "electron sea" (within which it is strongly-bound) to vaporize it, and that requires a lot of energy and leads to a high surface tension. Surface tension is inherently a two-phase property!


I think the contributing factors are:



```
Viscosity - liquid only, low energy change for motion, low viscosity
Surface tension - liquid-vapor, large energy change, high surface tension

```

It's beyond high school level, but I found Rowlinson and Widom's "Molecular Theory of Capillarity" gave very a good and thorough explanation of surface tension.


 2