The atomic number of an element corresponds to the number of protons in its nucleus. In this case, the element with an atomic number of 18 is argon (symbol: Ar).
To determine the group number and period of argon, we need to consult the periodic table. Argon is located in Group 18 (also known as Group 8A or Group 0) and Period 3 of the periodic table.
Group 18 elements are known as the noble gases or inert gases. They are characterized by having a full valence electron shell, making them stable and relatively unreactive. Argon, with its 18 electrons, has a full valence shell and exhibits these noble gas characteristics.
Period 3 of the periodic table includes elements with their outermost electrons occupying the third energy level.
In summary, the element with an atomic number of 18, argon (Ar), belongs to Group 18 and Period 3 of the periodic table.