Niobium and Tantalum have similar atomic size.why?
Due to lanthanide contraction. In the 4f-subshell, electrons are progressively filled from cerium to lutetium. As the shielding of 4f orbitals are not particularly effective though there is increasing nuclear charge the elements immediately following the lanthanide have atomic radii similar. Thus Niobium and Tantalum have similar atomic size.