What is Lanthanoid contraction ?

Explanation

The gradual decrease in the size of atoms or ions of lanthanides is called lanthanide contraction. This is due to the poor shielding effect on 4f electrons.
Consequences :
1) The net result of the lanthanide contraction is that the second and the third series exhibit similar radii (e.g.: Zr 160 pm Hf 159 pm) and have very similar physical and chemical properties much more than that expected on the basis of usual family relationship.
2) The decrease in metallic radius coupled with increase in atomic mass results in a general increase in the density of these .elements :