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 like, 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. | 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]]<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. | ||
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 [[Johannes Diderik van der Waals|J. D. van der Waals]]). Hence, a van der Waals radius r<sub>0</sub> may be estimated from ''b''/''N''<sub>A</sub> = 4π/3 r<sub>0</sub><sup>3</sup>. 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. | 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 [[Johannes Diderik van der Waals|J. D. van der Waals]]). Hence, a van der Waals radius r<sub>0</sub> may be estimated from ''b''/''N''<sub>A</sub> = 4π/3 r<sub>0</sub><sup>3</sup>. 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.<ref>A. Bondi, ''Van der Waals volumes and radii'', Journal of Physical Chemistry, vol. '''68''', p. 441 (1964)</ref> 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. | |||
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Revision as of 04:31, 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 like, 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.