The d-block consisting of Groups 3-12 occupies the large middle section of the periodic
table In these elements the inner d orbitals are progressively filled The f-block is placed
outside at the bottom of the periodic table and in the elements of this block, 4f and
5f orbitals are progressively filled Corresponding to the filling of 3d, 4d and 5d orbitals, three series of transition
elements are well recognised