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

Explanation

(i)N/A
(ii) N/A
(iii) In the elements belonging to Actionids series 5f, 6d and 7s orbitals have comparable enegies and the electrons present in these orbitals can take part in the bond formation to show higher oxidation states.