A It is filling up of 4f orbital before 5d orbital results in a regular decrease in atomic radii is called Lanthanoid contraction.
Consequences of lanthanoid contraction
(i) There is similarity in the properties of second and third transition series.
(ii) Separation of lanthanoids is possible due to lanthanide contraction.
(iii) It is due to lanthanide contraction that there is variation in the basic strength of lanthanide hydroxides. (Basic strength decreases from La(OH)3 to Lu(OH)3).
