As we move across 4f series, the atomic and ionic radii of lanthanoids show gradual decrease with increse in atomic number. This decrese in ionic size is called lanthanoid contraction.
As we move from one element to another in 4f series ( Ce to Lu) the nuclear charge increases by one unit and an additional electron is added into the same inner 4f sub shell. We know that 4f sub shell have a diffused shapes and therefore the shielding effect of 4f elelctrons relatively poor.hence, with increase of nuclear charge, the valence shell is pulled slightly towards nucleus. As a result, the effetive nuclear charge experienced by the 4f elelctorns increases and the size of Ln3+ ions decreases. Lanthanoid contraction of various lanthanoids is shown in the graph
As we from Ce3+ to Lu3+ , the basic character of Ln3+ ions decrease. Due to the decrease in the size of Ln3+ ions, the ionic character of Ln −OH bond decreases (covalent character increases) which results in the decrease in the basicity.
In the complete f - series only 10 pm decrease in atomic radii and 20 pm decrease in ionic radii is observed. because of this very small change in radii of lanthanoids, their chemical properties are quite similar.
The elements of the second and third transition series resemble each other more closely than the elements of the first and second transition series. For example