That's incorrect! Lanthanides and actinides are absolutely part of the periodic table. They are actually two separate series of elements that reside at the bottom of the table, separated from the main body.

Here's why they are placed that way:

* Electron Configuration: Lanthanides and actinides are characterized by the filling of the 4f and 5f orbitals, respectively. These orbitals are inner orbitals and don't contribute significantly to the chemical properties of these elements. To avoid cluttering the main body of the table, they are separated and placed below.

* Similar Chemical Properties: Elements within each series exhibit very similar chemical properties due to the gradual filling of these inner orbitals. This similarity makes it convenient to group them together.

So, in summary:

* Lanthanides (atomic numbers 57-71) and actinides (atomic numbers 89-103) are definitely part of the periodic table.

* They are placed below the main body due to their electron configuration and similar chemical properties.