Question

A 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 $Ti\left(IV\right)$ and $Zr\left(IV\right)$ ions are $0.68A^o$ and $0.74A^o$ respectively; but for $Hf\left(IV\right)$ ion the ionic radius is $0.75A^o$ which is almost the same as that for $Zr\left(IV\right)$ ion. This is due to:

• A. greater degree of covalency in compound of $H{f}^{4+}$
• B. lanthanide contraction
• C. difference in the coordination number of $Z{r}^{4+}$ and $H{f}^{4+}$ in their compounds
• D. actinide contraction

Answer 1

Answer: (B)

lanthanide contraction

Lanthanide contraction : The regular decrease in the size of lanthanide ions is known as lanthanide contraction. It is due to greater effect of the increased nuclear charge than that of the screening effect. Consequences of lanthanide contraction(a) It results in slight variation in their chemical properties which helps in their separation by ion exchange

(b) Each element beyond lanthanum has same atomic radius as that of the element lying above it in the group (e.g. $Zr$ 145 pm, $Hf$ 144 pm); $Nb$ 134 pm, $Ta$ 134 pm ; $Mo$ 129 pm, $W$ 130 pm).