Lanthanoid contraction: lanthanide contraction, also called lanthanoid contraction, in chemistry, the steady decrease in the size of the atoms and ions of the rare-earth elements with increasing atomic number from lanthanum (atomic number 57) through lutetium (atomic number 71).
The cumulative effect of the contraction of the lanthanoid series causes the radii of the members of third transition series to be similar to those of corresponding members of second series. Zr (160 pm) and Hf (159 pm) have identical radii. Due to their similar radii, they have same physical and chemical properties.
