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 the person 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.