Lanthanide Contraction is defined as the steady decrease in the size of lanthanide ions (M³⁺) with the increase in atomic number.

As we move down the group from left to right in a lanthanide series, according to the trend, the atomic no. increases and for every proton in the nucleus the extra electron goes to 4f orbital. Since the 4f orbital is too diffused to shield the nucleus effectively, there is a gradual increase in the effective nuclear charge experienced by the outer electrons. As a result, the attraction of the nucleus for the electrons in the outermost shell increases with the increase of atomic number, thus size decreases.

Consequence of Lanthanide Contraction:

1. Due to the very similar sizes of the lanthanides, it is difficult to separate them. However, due to lanthanide contraction their properties slightly vary in terms of the ability to form complexes. The variation in the properties can be utilized to separate them.

2. Due to the lanthanide contraction, size of M³⁺ions decreases and there is increase in covalent character in M–OH and hence basic character decreases.

3. The atomic radii of second row transition elements are almost similar to those of the third row transition elements because the increase in size on moving down the group from second to third transition elements is cancelled by the decrease in size due to the lanthanide contraction.