4 3 show an increase from the first (3d) to the second (4d) series
of the elements but the radii of the third (5d) series are virtually the
same as those of the corresponding members of the second series This
phenomenon is associated with the intervention of the 4f orbitals which
must be filled before the 5d series of elements begin The filling of 4f
before 5d orbital results in a regular decrease in atomic radii called
Lanthanoid contraction which essentially compensates for the expected
4