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 The decrease in metallic radius coupled with
increase in atomic mass results in a general
increase in the density of these elements