What are the causes of lanthanoids contraction ?

Explanation

(i) As we move from oen 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.
(ii) 4f sub shell have a diffused shapes and therefore shielding of 4f electrons relatively poor. Hence with increase of nuclear charge the valence shell is pulled slightly towards nucleus.
(iii) As a result the effective nuclear charge experienced by the 4f electrons increases and the size of `Ln^(3+)` ions decreases Lanthaoid contraction of various lanthanoids is shown in teh graph.