The steady decrease in size of lanthanide elements
and their ions is called lanthanide contraction.
In lanthanide elements, the differentiating electron
enters into antepenultimate 'f-' subshell.
(a) How does the entry of differentiating electron
into antepenultimate shell affect the atomic and
ionic sizes?
Densities of 6th period elements are double to
those of 5th period elements. Why?

Explanation

(a) In lanthanide elements, the newly added electron
enters into anti-penultimate shell keeping the
configurationn of penultimate and valence shells
same. The addition of electrons to inner shell
should result in increased screening effect. But
the 'f-' subshell has a very poor screening effect.
So the addition of electrons to the innerr 'f-"
subshell does not result in corresponding increase in
atomic size Therefore, the increasing nuclear
charge fron cerium to lutetium (71) results in
drastic decrease in atomi and ionic size.
(b) Between the 5th and 6th periods, the intervening
lanthanides increase the nuclear charge and
lanthanide contraction results in decrease of
atomic size. as a reuslt of this, mass of atom
increases and atomic size deceases. This results
in drastic increase in density of elements from
the 5th to 6th periods.