Chapter 4, Problem 4.109QP

Contrary to the generalized trend that atomic radius increases down a group, the atomic radius of Zr (159 pm) is actually larger than that of Hf (156 pm) even though the number of electrons increases from 40 (Zr) to 72 (Hf). Suggest a reason for this anomaly. (Hint: Many elements of the sixth period after Hf exhibit this same phenomenon in which their atomic radius is not significantly larger or is even smaller than that of the fifth period element above it.)