why the vanderwall radii are generally much larger than covalent radii?
The covalent radius is always smaller than the corresponding vander Waal's radius. This is because of the fact that in the formation of a chemical bond, the two atoms have to come closer to each other. This also explains why covalent bonds are much stronger than the vander Waal's forces. It is important to note that since the noble gases ordinarily do not form any covalent bond, in crystals of noble gases, no chemical forces are operating between the atoms. Hence the vander Waal's forces are the only attractive forces in these cases. In other words, the vander Waal's radii constitute the atomic radii of noble gases and since vander Waal's radii are larger than covalent radii, atomic radii of noble gases are largest in their respective periods (anomaly).