Give an explanation for each of the following observations :
(i) The gradual decrease in size (actinoid contraction) from element to element is greater among the actinoids than that among the lanthanoids (lanthanoid contraction).
(ii) The greatest number of oxidation states are exhibited by the members in the middle of a transition series.
(iii) With the same d-orbital configuration `(d^4) , Cr^(2+)` ion is a reducing agent but `Mn^(3+)` ion is an oxidising agent.