The element with atomic number 18 is argon (Ar). Argon belongs to the noble gases group, also known as Group 18 or Group 8A. The noble gases are located in the far-right column of the periodic table and include helium, neon, argon, krypton, xenon, and radon.
In terms of the period, the period number corresponds to the energy level or shell in which the element's valence electrons reside. For argon, its electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁶, indicating that it has its valence electrons in the third energy level or shell. Therefore, argon is in Period 3 of the periodic table.