Final answer:
The lanthanides and actinides are placed below the main body of the periodic table to maintain a compact form and because they have unique electron configurations with unfilled f-sublevels. The series include lanthanum and actinium due to their similar chemical behaviors.
Step-by-step explanation:
The lanthanides and actinides are known as the inner transition metals and belong to Periods 6 and 7 of the periodic table, respectively. These two series are grouped below the other elements due to practical reasons of representation and because they have unique electron configurations with unfilled f-sublevels, distinguishing them from the main group elements and the transition metals.
The lanthanides consist of elements with atomic numbers 57 through 71 (from cerium to lutetium) and the actinides consist of elements with atomic numbers 89 through 103 (from thorium to lawrencium).
Lanthanum (La) and actinium (Ac) are included in these series and are used to name them because they exhibit similar chemistry to the other members of their respective series, even though they are transition metals and do not have f-electrons in their ground state electron configurations.
The organisation in the periodic table helps in understanding chemical properties and trends among various groups of elements, with lanthanides and actinides making up the f-block elements due to their electron configurations. This positioning also aids in maintaining a more compact and manageable periodic table.