(a) (i)Lanthanoids show limited number of oxidation states viz., +2, +3 and +4 (dominant O.S. = +3) due to large energy gap between 4f and 5d subshells. The dominant O.S. of actinoids is also +3 but they also show a number of other oxidation states, e.g., uranium and plutonium show +3, +4, +5, +6 while neptunium shows +3, +4, +5, +7 oxidation states due to small energy gap between 5f, 6d and 7s subshells. This makes the chemistry of actinoids more complicated.
(ii) It is due to :
(a) comparatively smaller size of transition metal ions.
(b) their high ionic charges.
(c) availability of vacant d-orbitals
(b) 2MnO−4+6H++5SO2−3→2Mn2++5SO2−4+3H2O