Question

How do you account for the larger size of lanthanum than hafnium though they are successive elements?

Answer 1

Lanthanum is the first element of $3^{rd}$ series of transition elements. The differentiating electron goes into '5d' subshell. Though hafnium is present next to lanthanum, its atomic number is 72. That means, there is a large difference in nuclear charge. Though 14 electrons are added to the inner shell, the poor screening effect of $f$ subshell is not sufficient to balance the increased nuclear charge. Increase in effective nuclear charge results in a decrease in atomic radii. This makes the atomic size of hafnium smaller than lanthanum.