An increase in both atomic and ionic radii with atomic number occurs in any group of the periodic table and in accordance with this, the ionic radii of Ti (IV) and Zr (IV) ions are 0.68 A and 0.74 A respectively, but for Hf (IV) ion the ionic radius is 0.75 A, which is almost the same as that for Zr (IV) ion. This is due to
1
Greater degree of covalency in compounds of Hf4+
2
Lanthanide contraction
3
Difference in the co-ordination number of Zr4+ and4+ in their componds
4
actinide contraction.