This contraction is, of
course, similar to that observed in an ordinary
transition series and is attributed to the same
cause, the imperfect shielding of one electron
by another in the same sub-shell However, the
shielding of one 4 f electron by another is less
than one d electron by another with the increase
in nuclear charge along the series There is
fairly regular decrease in the sizes with
increasing atomic number The cumulative effect of the contraction of
the lanthanoid series, known as lanthanoid
contraction, causes the radii of the members
of the third transition series to be very similar
to those of the corresponding members of the
second series