Ionic Sizes

The trend already familiar from the lanthanoids repeats itself here: moving across the actinoid series, both the atomic size and the size of the $M^{3+}$ ion decrease steadily. This steady shrinkage is called the actinoid contraction, by direct analogy with the lanthanoid contraction.

The actinoid contraction, however, is not simply a smaller-scale repeat of the lanthanoid one — element to element, the contraction in the actinoid series is actually greater than in the lanthanoid series. This sharper contraction is a consequence of poorer shielding by the $5f$ electrons: because $5f$ orbitals shield the nuclear charge less effectively than $4f$ orbitals do, each successiv …