Final answer:
The lanthanides and actinides are placed separately on the modern periodic table due to their unique electron configurations and properties.
Step-by-step explanation:
The lanthanides and actinides are placed separately on the modern periodic table because they have similar electron configurations and properties. They are a group of elements known as the inner transition elements. The lanthanides, also known as the rare earth elements, are located in the f-block and have atomic numbers ranging from 57 (lanthanum) to 71 (lutetium).
The actinides are located below the lanthanides and have atomic numbers ranging from 89 (actinium) to 103 (lawrencium). They are characterized by their filling of the 5f sublevel. Both the lanthanides and actinides have unique electron configurations due to the filling of their f-orbitals.
Although lanthanum and actinium are transition metals, they are included in the lanthanide and actinide series due to their similarities to the other members of the series. Overall, the lanthanides and actinides are placed separately on the periodic table to emphasize their unique electronic structures and properties.