What is lanthanoid contraction ? What are the consequences of lanthanoid contraction?

Explanation

The steady decrease of atomic or ionic size from left to right in lanthanides, as the atomic number increases, is called 'Lanthanide contraction'.
In lanthanides, the differentiating electron enters the (n-2) f sub-shell. Due to their peculiar shapes, f orbitals do not provided proper shielding for the valence electrons from the nuclear attraction. Consequently the atomic or ionic size decreases gradually from left to right in lanthanides.
The decrease in contraction is more regular in `Ln^(+3)` ions than in Ln atom.
Consequences:
(1) Beacuse of the lanthanide contraction, the hardness, m.p, b.p, of the elements increase from Ce to Lu.
(2) There are more similarities between 4d and 5d series of elements, then between 3d and 4d series. The main reason is, 'lanthanide contraction'.
(3) The effect of lanthanide contraction is felt even in the post lanthanides. As a result, atomic sizes in the pairs of elements `Zr//Hf , Nb//Ts, Mo//W` are the same. So these pairs of elements possess similar properties. Zr and Hf resemble each other so much that their isolation is a quite difficult process.