# Why does Helium have such a low van der Waals coefficient but not as low a Lennard Jones coefficient?

Helium has the lowest polarizability of any atom, and therefore ought to have the smallest London dispersion force. Indeed, if you look at the van der Waals constant of helium, you find that it has the lowest value of $a$ by a lot. $a$ roughly corresponds to the amount attraction between particles.

The long-distance force between neutral particles can be roughly modelled by the Lennard-Jones potential. Even though Helium should have the weakest $r^{-12}$ term of any atom, if you look up the experimental values of the Lennard Jones coefficients, you find that they're the same for Helium as they are for the rest of the Nobel gases. What gives? Why is its Lennard Jones potential roughly the same when its London dispersion force is so weak? (Perhaps my source is just wrong?)

• Can you post your source? When I look up the values for $\sigma$ for the LJ potential, I find helium has the smallest of everything in the table, and is always smaller than the other noble gases. Keep in mind that that a small change in $\sigma$ can have a large change in the attraction because the attraction goes as $\sigma^6$ and repulsion $\sigma^{12}$. – jheindel Feb 9 '18 at 6:55
• Even your linked source shows that helium has a smaller value of $\sigma$. – orthocresol Feb 9 '18 at 21:30