In the transition series, starting from lanthanum `La(Z=57)`, the next element hafnium `Hf(Z=72)` has an atomic number of 72. Why do we observe this jump in atomic number?

Explanation

This is because after `La(Z=57)` filling of 4f-orbital starts which is complete at atomic number 71. These 14 elements, therefore,belong to f-block and placed separately at the bottom of the periodic table.