(a) The two consequences of lanthanide contraction are:
(i) Due to close similarity in electronic configuration and ionic radii, the lanthanides have identical chemical properties which makes their separation difficult.
(ii) Due to lanthanide contraction, the size of Ln3+ ions decrease regularly with increase in atomic number. According to Fajan's rule, decrease in size of Ln3+ ions increase the covalent character and decrease the basic character between Ln3+ and OH−ion in Ln(OH)3. Since the order of size of Ln3+ ions are La3+>Ce3+...>Lu3+.
(b) The element which shows largest number of oxidation states in 3d series is Manganese (Mn). It shows the following oxidation states +2,+3, +4,+5,+6 and +7 . It is because it contains the maximum no. of unpaired e−s in the outermost shell, i.e., 3d94 s2.