The factor responsible for the lanthanoid
contraction is somewhat similar to that observed
in an ordinary transition series and is attributed
to similar cause, i e , the imperfect shielding of
one electron by another in the same set of orbitals However, the shielding of one 4f electron by
another is less than that of one d electron by
another, and as the nuclear charge increases
along the series, there is fairly regular decrease
in the size of the entire 4f 
n orbitals