Here's why:
* Lanthanides: The elements Ce to Lu are known as the Lanthanides. They are part of the f-block elements, which are characterized by the filling of the f-orbitals.
* 4f sublevel: The 4f sublevel can hold up to 14 electrons. The lanthanides fill this sublevel sequentially, starting with Cerium (Ce) having one 4f electron and ending with Lutetium (Lu) having a full 4f sublevel with 14 electrons.
Therefore, the 4f sublevel is being filled by the elements Ce to Lu.
