The elements in which the last electron enters any one of the seven f-orbitals of their respective ante-penultimate shells are called f-block elements.

In all these elements, the s-orbital of the last shell (n) is completely filled, the d- orbitals of the penultimate (n -1) shell  is invariably contains zero or one electron but the f-orbitals of the ante-penultimate shell (n -2) shell (being lower in energy than d-orbitals of the penultimate shell) gets positively filled in. Hence, general outer shell electronic configuration of  f– block elements is (n –2) f ⁰⁻¹⁴ ns² , where n = 6, 7.

There are two series of f-block elements each containing 14 elements each. Therefore, there are 28 f-block elements in all in the periodic table. These elements are placed at the bottom of the periodic table.

The elements of  the first series (4 f – series), i.e. from lanthanum (₅₇La) to Lutetium (₇₁ Lu), which forms a part of sixth period are collectively called lanthanoids or lanthanides.

Since all the these elements follow lanthanum in the periodic table and closely resemble lanthanam (La) in their properties. Since all these elements occurs scarcely in the earth’s crust, therefore, these are also called rare earth elements. In lanthanoiuds , 4f orbitals are being progressively filled.

The second series (5 f-series) consist of elements from actinium (₈₉Ac) to lawrencium ( ₁₀₃Lr) and forms a part of incomplete seventh periiod. Since these elements follow actinium in the periodic table, therefore, these elements are also called actinoids or actinides.

In actinoids, 5f- orbitals are being progressively filled in and these elements resemble actinium in their properties.

All actinoids are radioactive elements the first four i.e. actinium (Ac), thorium (Th), protactium (Pa) and uranium (U) occur in nature while the remaining eleven elements from neptunium (₉₃Np) to lawrencium (₁₀₃Lr) are prepared artificially through nuclear reactions.

These eleven elements follow uranium in the periodic table and are prepared through nuclear reactions, therefore , these are also called transuranic or transuranium elements.

Lanthanoidd and actinoids which contitute the f block elements are also called inner transition elements because they form transitiion series with in the transition elements of d-block.