As 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^(4+) and Zr ^(4+)` ion are 68 Pm and 74 Pm respectivelt. But for `Hf ^(4+)` ion, the ionci radius is 75 Pm. Which is most same as that for `Zr ^(4+)` ion. This is due toA. greater degree of covalency in compound of `Hf ^(4+)`B. lanthanide contractionC. difference in co-ordination number of `Zr ^(4+) and Hf ^(4+)` in their compoundD. actinide contraction

As increase in both atomic and ionic radii with atomic number occurs i

1.	As 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^(4+) and Zr ^(4+)` ion are 68 Pm and 74 Pm respectivelt. But for `Hf ^(4+)` ion, the ionci radius is 75 Pm. Which is most same as that for `Zr ^(4+)` ion. This is due toA. greater degree of covalency in compound of `Hf ^(4+)`B. lanthanide contractionC. difference in co-ordination number of `Zr ^(4+) and Hf ^(4+)` in their compoundD. actinide contraction
Answer» Correct Answer - B In the same group atomic size increases ad atomic number increaes. But due to the lanthanide contraction atomic size of the elements just following Hf, Ta, W, Re, Os, etc. onwards are close to the element just above them in the respective group i.e. Zr, Nb, Mo, Tc, Ru etc. Due to the same size of Hf and Zr, Ta and Nb, W and Mo, Re and Tc, Os and Ru etc., the two properties and they occurs together in nture and are difficult to separate. This pair of elements is known as chemical twins. As a result of lanthanide contraction the elements of `2 ^(nd)and 3^(rd)` row transition series (d-block) resembles each other much more than the elements of `1^(st) and 2^(nd)` row trsnsition series. So they do not shwo close similarities.