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?

The steady decrease in size of lanthanide elements and their ions is

1.	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?
Answer» Solution :(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.