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

In lanthanum, 5d is more stable than 4f-subshell. Therefore, the 14 elements after La involve the filling of 4f-orbitals and this subshell gets completely filled at Lu. After Lu, the next electron enters the 5d-subshell in Hf (Z = 72) and it is placed in third transition series after La.