Van der Waals radius: definition, measurement, and applications

How it is measured

There is no single experimental procedure that uniquely defines a van der Waals radius. Common approaches include analysis of crystal structures (X‑ray or neutron diffraction) where center‑to‑center distances between nonbonded atoms yield sums of van der Waals radii, and thermodynamic methods that use gas properties such as the critical point. Computational fits to intermolecular potential energy minima and parameters from molecular mechanics force fields are also widely used.

Characteristics and interpretation

The van der Waals radius typically falls in the picometer scale and depends on the element and its electronic environment. It differs from covalent and ionic radii: covalent radii describe bonded atoms, while ionic radii apply to ions in lattices. Because the van der Waals radius is derived from macroscopic or structural data, it is an effective, context‑dependent quantity rather than a strict physical boundary.

Uses and examples

Van der Waals radii are used to model molecular shape and packing, estimate nonbonded contact distances in crystal engineering, and represent excluded volume in simulations. They are important in medicinal chemistry for analyzing steric clashes, in materials science for predicting porosity and adsorption, and in surface science where adsorbate spacing matters.

Distinctive facts and limitations

• The same atom can have different published van der Waals radii depending on the method and dataset used.
• Sum of van der Waals radii is often used as an empirical predictor of the shortest expected distance between two nonbonded atoms.
• The term is named after Johannes Diderik van der Waals, who emphasized finite molecular size and intermolecular forces in his work on the equation of state.

Further reading

For practical use, tables of van der Waals radii compiled from crystallography and force‑field parametrizations are widely available; consult a reliable handbook or database when precise values are required. For an introduction and additional sources, see further reading on van der Waals radii.