Freeze like a (noble) gas until it forms a solid. Half the distance between close non-bonded nuclei. Doesn't work for Helium, as it is never ever solid. Also van der walls radius is very smaller than covalent radius, suggesting there's a correlation between kinetic energy, and atomic radius.

This is basically no two electrons can be at the exact same place. Thus no two electrons can share the exact same 4 quantum #s. This is called the Pauli Exclusion Principle.