How would you account for the following ?
(i) Many of the transition elements are known to form interstitial compounds.
(ii) The metallic radii of the third `(5d)` series of transition metals are virtually the same as those of the corresponding group members of the second `(4d)` series
(iii) Lanthanoids from primarily +3 ions, while the actinoids +1 usually have higher oxidation state in their compounds ,`+4 or even +6being typical.

Answer

(i) It is because transition elements have voids of suitable size in which small atoms like `H,C,B` etc, can fit into forming interstitial compounds.
(ii) Due to lanthanoid contraction, effective nulcear charge remains almost same therefore , mettalic radii are nearly same
(iii) In Actinoids `5f,6d` and 7s orbitals have comparable energies and electrons fromt these orbitals can take part to show higher oxidation states.