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Why do zirconium and hafnium have nearly identical atomic radii?

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A line graph showing electron probability distributions for gadolinium, with separate curves for 4f, 5s, 5p, 5d, and 6s orbitals. The x-axis shows distance from the nucleus in atomic units and the y-axis shows probability P2(r). — rare-earth element Electron probabilities, P²*(r)*, for the 4f, 5s, 5p, 5d, and 6s electrons of gadolinium.

The nearly identical atomic radii of zirconium and hafnium are primarily due to the lanthanide contraction.12

In the periodic table, hafnium (atomic number 72) immediately follows the lanthanide series, which consists of elements with atomic numbers 57 through 71. As these elements are filled, electrons are added to the 4f orbitals. Because 4f electrons provide very imperfect shielding of the nucleus's positive charge, the effective nuclear charge experienced by outer electrons increases steadily across the series.1

This phenomenon causes a consistent decrease in atomic and ionic radii throughout the lanthanides. By the time the series concludes, the atoms have become significantly smaller than would otherwise be expected. Consequently, when hafnium is reached, its atomic radius is reduced to a size nearly identical to that of the lighter zirconium (atomic number 40), which sits directly above it in Group 4.12