Why the properties of third transition series are very similar to second transition series ? Or Why the second and third members in each group of the transition elements have very similar atomic radii ?

Explanation

In the third transition series after lanthanum, there is lanthanoid contraction .Due to this contraction, the size of any atom of the third transition series is almost the same as that of the element lying just above in the second transition series. This leads to similarity in their properties.