Electronic configuration of Lanthanoids:
We know that the electrons are filled in different orbitals in the order of their increasing energy in accordance with Aufbau principle. As per this rule after filling 5s,5p and 6s and 4f level begin to fill from lanthanum, and hence the expected electronic configuration of Lanthanum(La) is [Xe] 4f 1 5d0 6s2 but the actual electronic configuration of Lanthanum is [Xe] 4f 0 5d1 6s2 and it belongs to d block. Filling of 4f orbital starts from Cerium (Ce) and its electronic configuration is [Xe] 4f 1 5d1 6s 2 . As we move from Cerium to other elements the additional electrons are progressively filled in 4f orbitals as shown in the table.
In Gadolinium (Gd) and Lutetium (Lu) the 4f orbitals, are half-filled and completely filled, and one electron enters 5d orbitals. Hence the general electronic configuration of 4f series of elements can be written as [Xe] 4f 2 −14 5d0−1 6s2