The pair , that referred as 'chemical twins' is

Options

A. Ac, Cf
B. Hf,Ta
C. Tc,Re
D. La,Ac

Answer

C

Explanation

Due to the presence of f-electrons , the nucleus is not screened effectively . Thus , the atomic readius of elements of second transition series ( ie, Nb, Mo, Tc etc ) is similar to that of corresponding third transition series elements (ie , Ta , W , Re ) .Hence , Nb-Ta,Mo-W or Tc-Re are called chemical twins .