The ionic radius of an atom is the distance from the center of an atom to its valence electrons.

With increase in atomic number the ionic radii of actinides decreases. This is because as we move from left to right in a period, atomic number increases and hence the number of protons in nucleus also increases. As a result, repulsion between protons becomes more significant than that between neutrons and electrons. Thus, in heavier elements (with higher atomic numbers) there are more protons than electrons and thus larger repulsion between protons leads to smaller ionic radius.