The octet rule is a simple chemical guideline that allows chemists to predict the placement of electrons around the nucleus (electron orbitals), the identification of electrons added or lost during chemical reactions, and the chemical reactivity of atoms based upon their particular electron configuration. The octet rule is used when drawing Lewis dot structures and diagramming electron configurations. Although the octet rule does not work for all elements, it does work well for such elements as carbon, bromine, chlorine, iodine, and oxygen.
The octet rule is used to describe the attraction of elements toward having, whenever possible, eight valence-shell electrons (four electron pairs) in their outer shell. Because a full outer shell with eight electrons is relatively stable, many atoms lose or gain electrons to obtain an electron configuration like that of the nearest noble gas. Except for helium (with a filled 1s shell), noble gases have eight electrons in their valence shells.
The electron configurations of sodium and chloride ions—the components of table salt (NaCl)—provide a useful insight to the octet rule.
Sodium (Na) with an electron configuration of 1s 22s 22p 63s 1 sheds its outermost 3s electron and, as a result, the Na+ ion has an electron configuration of 1s 22s 2 2p 6. This is the same electron configuration as neon, a noble gas (i.e., highly stable and relatively nonreactive).
Chorine (Cl), on the other hand, has an electron configuration of 1s 22s 22p 63s 23p 5. Chorine needs one electron to fill its outermost third shell with eight electrons. When chorine takes on the electron shed by sodium then the Cl– ion’s electron configuration becomes 1s 22s 22p 63s 23p 6 and this is the same configuration as argon, the nearest noble gas.
In general, the octet rule works for representative metals (Groups IA, IIA) and nonmetals, but not for the transition, inner-transition or post-transition elements. These elements seek additional stability by having filled half-filled or filled orbitals d or f subshell orbitals. The octet rule does not, however, accurately predict the electron configurations of all molecules and compounds. Not all nonmetals, nor metals, can form compounds that satisfy the octet rule. As a result, the octet rule must be used with caution when predicting the electron configurations of molecules and compounds. For example, some atoms violate the octet rule and are surrounded with more than four electron pairs.