Van der Waals radius: Difference between revisions
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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 | 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-chemically bound, atoms. The concept was introduced by [[Linus Pauling]]<ref>L. Pauling, ''The Nature of the Chemical Bond'', 3rd edition, Cornell University Press, Ithaca, NY (1960)</ref>, who extracted the van der Waals radii mainly from [[lattice spacing]]s in [[molecular crystal]]s. As an example Pauling quotes a crystal consisting of Cl<sub>2</sub>-molecules. Obviously, the Cl-atoms of neigboring molecules touch in a Cl<sub>2</sub>-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 Cl<sub>2</sub>-molecule. | ||
Revision as of 03:44, 13 September 2007
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-chemically bound, atoms. The concept was introduced by Linus Pauling[1], 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/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.[2] 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.