Lanthanoid contraction refers to the overall decrease in atomic and ionic radii from lanthanum to lutetium. This is a unique feature in the chemistry of the lanthanoids.

This contraction is similar to that observed in an ordinary transition series and is attributed to the imperfect shielding of one electron by another in the same sub-shell. However, the shielding of one 4f electron by another is less than that of one d electron by another. As the nuclear charge increases along the series, there is a fairly regular decrease in the size of the entire 4f orbitals.

The cumulative effect of this contraction has far-reaching consequences in the chemistry of the third transition series elements. The net result is that the second and third d-series exhibit similar radii. For example, Zr has a radius of 160 pm while Hf has a radius of 159 pm.

Due to this, the elements of the second and third transition series have very similar physical and chemical properties, much more than expected on the basis of usual family relationship. The almost identical radii of Zr and Hf account for their occurrence together in nature.

Another consequence is the difficulty faced in separation of such elements due to their similar properties and radii.