Editing Van der Waals radius (section)

=== Polarizability ===

The [[polarizability]] ''α'' of a gas is related to its [[electric susceptibility]] ''χ''{{sub|e}} by the relation
<math display="block">\alpha = {\varepsilon_0 k_{\rm B}T\over p}\chi_{\rm e}</math>
and the electric susceptibility may be calculated from tabulated values of the [[Permittivity|relative permittivity]] ''ε''{{sub|r}} using the relation ''χ''{{sub|e}}&nbsp;= ''ε''{{sub|r}}&nbsp;−&nbsp;1. 
The electric susceptibility of helium ''χ''{{sub|e}}&nbsp;= {{val|7|e=-5}} at 0&nbsp;°C and 101.325&nbsp;kPa,<ref>Kaye & Laby Tables, [http://www.kayelaby.npl.co.uk/general_physics/2_6/2_6_5.html Dielectric Properties of Materials].</ref> which corresponds to a polarizability ''α''&nbsp;= {{val|2.307|e=-41|u=C⋅m<sup>2</sup>/V}}. 
The polarizability is related the van der Waals volume by the relation
<math display="block">V_{\rm w} = {1\over{4\pi\varepsilon_0}}\alpha ,</math>
so the van der Waals volume of helium ''V''{{sub|w}}&nbsp;= {{val|2.073|e=-31|u=m3}}&nbsp;= 0.2073&nbsp;Å{{sup|3}} by this method, corresponding to ''r''{{sub|w}}&nbsp;= 0.37&nbsp;Å.

When the atomic polarizability is quoted in units of volume such as Å{{sup|3}}, as is often the case, it is equal to the van der Waals volume. 
However, the term "atomic polarizability" is preferred as polarizability is a precisely defined (and measurable) [[physical quantity]], whereas "van der Waals volume" can have any number of definitions depending on the method of measurement.