F – Block Elements
The elements in which the differentiating electron enters the anti penultimate energy level i.e. (n – 2)f, are called f – block elements. These are often called as inner transition elements or rare earth elements. The differentiating electron in transition elements may enter either 4f or 5f orbitals based upon which they are differentiated into lanthanides and actinides.
In lanthanides the differentiating electron enters 4f orbital. These are cerium to lutetium. The name lanthanides is because they come immediately after lanthanum.
Actinides In actinides the differentiating electron enters 5f orbitals. These are thorium to lawrencium. These elements come immediately after actinium.
General electronic configuration of f – block elements is (n–2)f1–14(n–1)d0–1ns2