How can we measure the radius for noble gases?

For noble gases, the radius is measured as van der Waals radius because these atoms are held together by weak van der Waals forces.

In the case of inert or noble gases, the shells are completely filled with electrons, and they are stable.
As a result of this, due to the completely filled electronic configuration, the electronic repulsion in the noble gases are high.
So the electrons tend to be away from each other to minimise this repulsion, leading to an increased atomic size of the noble gases.
The atomic radius of the noble gases is determined by van der Waals radius, which is comparatively larger than ionic or covalent radii.

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