The elements in which the additional electron enters in (`n-2`) `f` orbitals are called inner transition elements or `f`-block elements.

`text(Position in the periodic table)`: The lanthanides resemble yttrium in most of their properties. So, it became necessary to accomodate all the fifteen elements together at one place. This has been done by placing the first element, lanthanum below yttrium and placing the remaining fourteen elements separately in the lower part of the periodic table.

`text(Lanthanide series)` (`Z = 58-71`)(`Ce - Lu`)

`text(Actinide series)` (`Z = 90- 103`)(`Th - Lw`)