Explain why the noble gas Kr does follow the general trend for covalent radii
Since the increased atomic radius causes a decrease in the electric interaction between electrons and nucleus, the heavier noble gas elements, such as Kr and Xe can form compounds with other elements. (KrF2, XeF4, ... &amp;c.)

The covalent radius is quite meaningless in for elements such as Ar and Ne since its compounds seldom exist, but Kr and Xe compounds can occur with relatively high stability, so its 'covalent radius' can be meaningful.

That 0.109nm is probably a covalent radius.

Something like measuring a Kr-F bold length in KrF2 and then subtracting the covalent radius of a fluorine atom.

Trivially, the Van der Waals radius tends to be bigger than the covalent one, the noble gas atom radius seems as if it does not follow the trend, but from Krypton which the covalent radius is measurable, the data follows the general trend in periodic table.