Zirconium covalent radius

This similarity of radii in the fifth- and sixth-period transition elements can be explained in terms of the lanthanide contraction. Between lanthanum and hafnium in the sixth period are 14 lanthanide elements (cerium to lutetium), in which the 4/ subshell fills. The covalent radius decreases slowly from cerium (165 pm) to lutetium (156 pm), but the total decrease is substantial. By the time the 4/subshell is complete, the covalent radii of the transition elements from hafnium on are similar to those of the elements in the preceding row of the periodic table. Therefore, the covalent radius of hafnium (146 pm) is approximately the same as that of zirconium (145 pm). 

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