Group 18 elements (helium, neon, and argon are shown) have a full outer, or valence, shell. A full valence shell is the most stable electron configuration. Elements in other groups have partially-filled valence shells and gain or lose electrons to achieve a stable electron configuration. read more
Elements in this group have nearly complete outer electron shells containing seven electrons. These elements are called the halogens. They are the most reactive elements and easily form compounds with other elements. read more