Atoms often gain, lose or share electrons trying to reach the same number of electrons as the noble gases closest to them on the periodic table. Noble gases have very stable electron arrangements, as revealed by their high ionization energies, their low affinity for additional electrons, and their general lack of chemical reactivity. Since all noble gases (with the exception of He) have eight valence electrons, many atoms that undergo reactions also end up with eight valence electrons. This observation has given rise to a pattern known as the octet rule: atoms tend to gain, lose or share electrons until they are surrounded by eight valence electrons. An octet of electrons consists of s- and p-filled subshells of an atom. In terms of Lewis symbols, an octet can be visualized as four pairs of valence electrons arranged around the atom.
