Question

Where are the lanthanides and actinides placed in the periodic table? Why?

Answer 1

You mean to ask why the lanthanides and actinides are placed in a separate section of the Periodic Table.

Explanation:

Both the lanthanide and actinide metals are LARGE, Lewis-acidic metals, in which the `f` subshell is progressively filled. The Group 3 metals, scandium, yttrium, and lanthanum, are often considered in this category, because they share similar chemistry.

Given the properties of the `"f-orbitals"` (which are diffuse, and offer poor electronic shielding), the f element metals usually give `M^(2+)`, and `M^(3+)` oxidation states, and are marked by extreme Lewis acidity. Their chemistry is the province of specialists.