Question

The size of $Nb$ is very similar to:

• A. $La$
• B. $Hf$
• C. $Ta$
• D. $W$

Answer 1

The correct option is C $Ta$

Owing to the poor shielding of nuclear charge (nuclear attractive force on electrons) by $4f$ electrons; the $6s$ electrons are drawn towards the nucleus, thus resulting in a smaller atomic radius. This contraction is known as Lanthanide Contraction. Due to lanthanide contraction, the normal size increase $Sc\to Y\to La$ disappears after Lanthanides. Thus pairs of elements such as $Zr/Hf,Nb/Ta,Mo/W$ are almost identical in size. Therefore $Nb$ and $Ta$ have similar size.