Hello,

In actinoids, 5f orbitals are filled. These 5f orbitals have a poorer shielding effect than 4f orbitals (in lanthanoids). Thus, the effective nuclear charge experienced by electrons in valence shells in case of actinoids is much more that than experienced by lanthanoids. Hence, the size contraction in actinoids is greater as compared to that in lanthanoids.

The interstitial compounds are the compounds that are formed when an atom with a small radius sits in an interstitial hole in a metal lattice.

The example of some of the small atoms that form interstetial compounds are: Hydrogen, Boron, Carbon and Nitrogen

Best Wishes.