van der Waals coefficients for helium and neon

The values of the coefficient $b$ for helium and neon are


$$\begin{array}{cc}
\hline
\text{Element} & b\text{ / }\mathrm{dm^3\ mol^{-1}} \\
\hline
\ce{He} & 2.38 \times 10^{-2} \\
\ce{Ne} & 1.67 \times 10^{-2} \\
\hline
\end{array}$$
(source: Atkins & de Paula, *Physical Chemistry*, 9th ed., p 916)


This doesn't make sense, however, considering that neon should have a larger atomic radius than helium. Why is $b\_\ce{He} > b\_\ce{Ne}$?




The question intregues me so I looked up the $a$ and $b$ values in Wikipedia on the web page [Van der Waals constants](https://en.wikipedia.org/wiki/Van_der_Waals_constants_(data_page)). This doesn't answer the question, but adds more data to focus the question. 


$$\begin{array}{cc}
\hline
\text{Element} & a\text{ / }\mathrm{L^2 \ bar\ mol^{-2}} & b\text{ / }\mathrm{L\ mol^{-1}}\\
\hline
\ce{He} & 0.0346 & 0.0238 \\
\ce{Ne} & 0.2135 & 0.01709 \\
\ce{Ar} & 1.355 & 0.03201 \\
\ce{Kr} & 2.349 & 0.03978 \\
\ce{Xe} & 4.250 & 0.05105\\
\ce{Rn} & 6.601 & 0.06239 \\
\hline
\end{array}$$


A plot of the Van der Waals $a$ and $b$ values.
[![enter image description here](https://i.stack.imgur.com/2v4YD.jpg)](https://i.stack.imgur.com/2v4YD.jpg)


So is the b value for He high, or is the b value for Ne low ?!?


I also looked up for each element what was listed for the covalent radius and the Van der Waals radius on each elements individual Wikipedia webpage. 


$$\begin{array}{cc}
\hline
\text{Element} & Covalent\text{ (pm) } & Van~der~Waals \text{ (pm)}\\
\hline
\ce{He} & 28 & 140 \\
\ce{Ne} & 58 & 154 \\
\ce{Ar} & 106±10 & 188 \\
\ce{Kr} & 116±4 & 202 \\
\ce{Xe} & 140±9 & 216 \\
\ce{Rn} & 150 & 220 \\
\hline
\end{array}$$


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