Van der Waals radius

in chemistry, a measure for the size of an atom that is not chemically (ionically or covalently) bound; half the closest distance of two equal, non-covalently bound, atoms

The Van der Waals radius is the radius of an atom that is seen as a hard sphere. It is named after someone who realized that atoms were not points, but actual objects. It can be found by the critical point and the physical properties of a gas. It also can be found by a molecular crystal.