The f–block elements of the periodic table consists of those elements whose atoms or ions have valence electrons in f–orbitals of anti–penultimate shell. f–block elements are also termed as inner transition elements as they are transition elements within the transition elements i.e., d–block elements. The general electronic configuration of the f–block elements is given as
(n − 2)f ^{1 − 14}(n − 1)d^{0 − 1}ns^{1 − 2}.

Classification of f–block elements:
Depending upon whether the last electron enters a 4f orbital or a 5f orbital, the f–block elements have been divided into two series as lanthanides and actinides.

Lanthanides:
The elements in which the last electron enters one of the 4f orbitals are called 4f–block elements or first inner transition series. These are also called lanthanides or lanthanons, because they come immediately after lanthanum. The 14 lanthanides cerium (Ce, Z=58) through lutetium (Lu, Z=71) lie between lanthanum (La, Z=57)and hafnium (Hf, Z=72) in the third d–block transition series.

Actinides:
The elements in which the last electron enters one of the 5f orbitals are called 5f–block elements or second inner transition series. These are also called actinides or actinons or actinoids because they come immediately after actinium. These actinoids are radioactive in nature and lie below the lanthanides.

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