What is the van der Waals radius?
by 
wickedwillie
ANSWERS: 3
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The van der Waals radius is the intermolecular distance in a molecular cristal. Let's say we have an iodine molecule (I-I) If it is a gas you just have I-I units floating around. The distance of the nuclei divided by two is their atomic radius. If they form a solid you still have molecules with covalent bonds, and between those bonds van der Waals bonds. The structure would look like this: I-I ... I-I ... I-I ... I-I (acually three dimensionally) The van der Waals radius is the "...", the distance between two molecules or precisely the distance beween the nuclei of non-bonding atoms.
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The van der Waal's radius is the distance between two atom centers when the force required to move the atoms closer to each other becomes positive (i.e., repulsive). It is the effective radius of an atom, as atoms are somewhat compressible or polarizable due to the majority of their volume being electron orbital clouds. The vdW radius is also known as the "hard sphere" radius, that is, it gives the size of an atom when it acts like a hard sphere. Remember, though, that at the atomic scale the world is very different (i.e., a quantum mechanical world, not Newtonian). "Hard sphere" is an analogy to something at our scale and is not a true description of an atom.
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The half of the distance between the nuclie of the two non bonded atoms of adjacantly placwd molucules is called vanderwall radius.
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