In 1916, Gilbert Lewis proposed a model that explained many observed facts about chemical bonding and chemical reactions. He pointed out that noble gases extremely stable and unreactive due to their eight electronic configuration.

The octet rule refers to the tendency of atoms to prefer to have eight electrons in the valence shell. When atoms have fewer than eight electrons, they tend to react and form more stable compounds. Atoms may lose, gain or share their valence electrons to achieve eight electronic configuration. Thus an atom will continue to form bond until it achieves eight electrons in the outermost shell. When discussing the octet rule, we do not consider d or f electrons. Only the s and p electrons are involved in the octet rule, making it useful for the main group elements (elements not in the transition metal or inner-transition metal blocks); an octet in these atoms corresponds to an electron configurations ending with s2p6.

Periodic chart helps us to identify the number of valence electrons of all the elements. On the periodic table Group IA, IIA and IIIA elements main group elements and contain metals only. Since metals are located on the left hand side of the periodic chart, metals usually have one two or three valence electrons. Metals tend to lose those electrons to gain previous noble gas configurations. Similarly, main group elements VA, VIA and VIIA elements are nonmetals only. Nonmetals gain extra valence electrons and achieves the next noble gas configuration in that period. Therefore, metals lose electrons equal to their valence electrons and form positively charged species. Nonmetals gain 8- valence electrons and form negatively charged species. Positively charged substances are called cations and negatively charged substances are called anions. By gaining or losing one, two or three electrons an atom forms an ion with a completely filled outer shell of electrons.

The charge on main group ions can be predicted from the position in the periodic table. For metals in group 1A, IIA and IIIA, the group number=charge on the cation. For nonmetals in group VA,VIA and VIIA, anion charge =8-group number. Main group elements are specially stable when they possess an octet of electrons. The charge of the ion is written as superscript. Find below the complete charge table for all the metals and nonmetals on the periodic table. Hydrogen and Boron are exception to the octet rule. Hydrogen needs 2 electrons and boron requires minimum six electrons to satisfy. later we will find some atoms may contain more than eight electrons.

Ions are written as Lewis symbol by showing their valence electrons as follows.