Final answer:
Zr (zirconium) and Hf (hafnium) are difficult to separate due to their similarities in electron configuration and chemical properties. Both belong to the transition metals and have similar electronic structures. The close resemblance in their chemistry and physical properties complicates their separation.
Step-by-step explanation:
The difficulty in separating Zr (zirconium) and Hf (hafnium) lies in their similarities in electron configuration and chemical properties. Both Zr and Hf belong to the transition metals and have very similar electronic structures. Zr belongs to the 4d transition series and Hf to the 5d transition series. However, due to the filling of the 4d and 5d orbitals, the energy difference between them is very small, making it challenging to separate them.
For example, both Zr and Hf form complex ions with ligands, and the complexes they form exhibit similar colors and properties. The similarities in their chemistry and physical properties make it difficult to separate them.