The element in Period 3 and Group 18 is Argon (Ar).
The element in Period 3 and Group 18 (also known as Group 8A or Group 0) is Argon (Ar).
Argon is a noble gas, and it is the third element in the third period of the periodic table.
Noble gases are characterized by their stable electron configurations and generally do not readily form compounds with other elements due to their filled outer electron shells.