Pyrolusite ore (MnO2) is used for the preparation of potassium permanganate Both the
dichromate and the permanganate ions are strong oxidising agents The two series of inner transition elements, lanthanoids and actinoids constitute
the f-block of the periodic table With the successive filling of the inner orbitals, 4f, there
is a gradual decrease in the atomic and ionic sizes of these metals along the series
(lanthanoid contraction)