The lanthanoid contraction causes the radii of the third transition series elements to be very similar to those of the corresponding second series elements. The nearly identical radii of Zr (160 pm) and Hf (159 pm), resulting from the lanthanoid contraction, lead to their occurrence together in nature and pose challenges in their separation.
How does the lanthanoid contraction impact the sizes of the third transition series elements, and what is the consequence of the almost identical radii of Zr and Hf?
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The lanthanoid contraction influences the sizes of the third transition series elements (from Hf to Hg). Due to the lanthanoid contraction’s effect on Zr and Hf, these elements have almost identical radii. This similarity arises because the addition of electrons to Zr and Hf involves the filling of 4d and 5p orbitals, respectively. The lanthanoid contraction mitigates the anticipated increase in size, resulting in Zr and Hf having nearly indistinguishable atomic and ionic radii. This phenomenon has practical consequences, making separation of Zr and Hf challenging in chemical processes like nuclear reactors, where Hf must be removed from Zr to prevent neutron absorption.