How would you account for the following : (i) The metallic radii of the third (5d) series of transition metals are virtually the same as those of the corresponding group members of the seconds (4d) series.
(ii) There is a greater range of oxidation states among the actinoids than among the lanthanoids.

Explanation

(i) Many transition elements and their compounds act as catalysts . The catalytic activity is due to their ability to exhibit multiple oxidation states . For example , `V_(2)O_(5)` in contact process and finely divided iron in Haber process.
(ii) This is due to the intervention of the 4f orbitals which must be filled before the 5d series of element begin . the filling of 4f before 5d orbitals results in a regular decrease in atomic radill Lanthanoid contraction.
(iii) This is due to the fact that 5f , 6f and 7s levels are of comparable energies.