Van der Waals radius

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 like, non-chemically 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. Obviously, the Cl-atoms of neigboring 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 Cl₂-molecule.

With regard to the origin of the term: In the thermodynamic van der Waals equation for the state of a gas appears a parameter b. This parameter divided by Avogadro's constant 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/N_A = 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.