Question

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

Answer 1

This is because after `._(57) La`, filling of 4 f-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.