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 correponding group members of the second (4d) series.
(iii) Lanthanoids from primarily +3 ions, while the actinoids usually have higher oxidation states in their compounds , +4 or even +6 being typical.

Explanation

Many of the transition elementare known to form interstial compounds. Transition elements form intensitial compounds as they are capable of entrapping small atoms like H,C or N in the interstitial sites in their crystal Lattice.
(ii) The metallic radii of the third (5d) series of the transition metals are virtually the same as those of the corresponding group memebers of the second (4d) series. This is due to filling 4f orbitals which have poor shielding effect or due to lanthanoid contraction.
(iii) This is attributed to the fact that 5f, 6d and 7s levels are of comparable energies in case of actionoids. Hence all these three subshells can participate on bonding.