Argon (Ar) is an inert gas belonging to Group 18 of the periodic table. It has a completely filled outer shell with eight electrons, making it stable and unreactive. Therefore, argon does not gain or lose electrons to achieve a full outer shell. Elements in Group 18, including argon, already possess a full complement of electrons in their valence shell, which is known as an octet configuration. This stable electron configuration is similar to the noble gas configuration, and it is the reason why elements in this group are generally unreactive or inert.