Lanthanides:
* Position: The 14 elements from Lanthanum (La) to Lutetium (Lu) occupy the first row of the f-block.
* Electronic configuration: These elements have their last electron entering the 4f-orbital.
Actinides:
* Position: The 14 elements from Actinium (Ac) to Lawrencium (Lr) occupy the second row of the f-block.
* Electronic configuration: These elements have their last electron entering the 5f-orbital.
Therefore, the names "lanthanides" and "actinides" are derived from their first members, Lanthanum and Actinium, respectively.
In summary:
* The f-block elements are called lanthanides and actinides due to their position in the periodic table and the fact that their differentiating electrons enter the f-orbitals.
* The names are based on the first element in each row of the f-block: Lanthanum for the lanthanides and Actinium for the actinides.
