Lanthanides and actinides are placed below the Periodic Table because of their unique physical and chemical properties.
The elements that have been removed from period 7 and placed below the periodic table are called the "lanthanides" and "actinides". These elements are also known as the "rare earth elements" and they are placed separately to conserve space on the periodic table.
The lanthanides are a series of elements that are placed at the bottom of the periodic table. They are located in the f-block, specifically in the period below the main body of the periodic table. The lanthanides have atomic numbers 57-71.
The elements that are placed outside of the periodic table are called transactinides. These elements are usually placed in a separate row below the main table and are synthetic elements that do not occur naturally on Earth.
Astatine is placed in group-17 in the Periodic Table. Its symbol is At.
Compounds are not placed in the periodic table.
Actinoids are placed in a separate row under the periodic table.
At the bottom of the Periodic Table
If new elements are placed, then they would be appended in the periodic table. It won't disturb existing periodic table.
Metals are placed at the left. Non-metals are placed at right in periodic table.
CO3 is not placed in periodic table. Only elements are arranged in periodic table.
The bottom section of the periodic table is for the lanthanides and actinides, which are also known as the inner transition metals. These elements are placed below the main body of the periodic table to keep it compact, as they would otherwise disrupt the table's organization.
The f-block elements are placed at the bottom of the periodic table because if they were placed where they belong the periodic table would be too big to fit on a normal sheet of paper.