i. The last electron in the f-block elements enters into (n-2) f-orbitals, i.e., inner to the penultimate energy level and they form a transition series within the transition series (d-block elements).
Hence, the f-block elements are known as inner transition series.
ii. There are two series of inner transition elements:
a. Lanthanoids (atomic number 58–71)
b. Actinoids (atomic number 90–103).