Question

The pairs that have almost the same atomic and ionic radii are:

• A. $V,Nb$
• B. $Ti,Zr$
• C. $Zr,Hf$
• D. $Co,Ni$

Answer 1

Answer: (C), (D)

The correct options are
C $Co,Ni$
D $Zr,Hf$

Due to lanthanide contraction elements of $V$ and $Vl$ periods have the same atomic and ionic sizes. Thus $Zr$ ($V$ period) and $Hf$ ($Vl$ period) have the same sizes. $TI\left(lV\right)$ and $Zr\left(V\right)$ as well as $V\left(Vl\right)$ and $Nb\left(V\right)$ have different sizes as there is no intervening lanthanide contraction. $Co$ and $Ni$ are successive metals and the increase in nuclear charge in $Ni$ is balanced by the screening effect of the one added $d$-electron. Hence they have the same radii.