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Van der Waals radius
From Citizendium, the Citizens' Compendium
In chemistry, a van der Waals radius is a measure for the size of an atom that is not chemically (ionically or covalently) bound. In general a van der Waals radius is defined as half the closest distance of two equal, non-covalently bound, atoms. The concept was introduced by Linus Pauling, who extracted the van der Waals radii mainly from lattice spacings in molecular crystals. As an example Pauling quotes a crystal consisting of Cl2-molecules. The Cl-atoms of neighbouring molecules touch in a Cl2-crystal. Half the distance between two touching chlorine atoms is the van der Waals radius of the Cl-atom. This radius is usually larger than the covalent radius, which for the Cl-atom is half the distance between the two Cl nuclei in the same Cl2-molecule.
With regard to the origin of the term: The thermodynamic van der Waals equation for a fluid contains the parameter b. This parameter divided by Avogadro's constant NA is the volume of the particles constituting the gas (these were seen as hard spheres by J. D. van der Waals). Hence, a van der Waals radius r0 may be estimated from b/NA = 4π/3 r03. However, since gases of stable atoms are rare, so that thermodynamic b values usually pertain to stable molecules, this determination of the van der Waals radius of an atom is not of practical interest.
A much used list of values for non-metallic elements was derived by Bondi. Recommended van der Waals radii are used in computer programs that draw space-filling models of molecules. Also molecular modelling programs that give structures of (biological) macromolecules use these data.