Lewis Symbols and the Octet Rule

Chemical bonding involves the sharing or transfer of valence electrons between two or more atoms leading to the lowering of potential energy — a contributing factor for bond formation.

The process of bond formation using valence electrons is explained by the Lewis model.

In the Lewis model, each valence electron of an atom from the s- or p-block is symbolized as a dot surrounding the abbreviation of the element, called a Lewis symbol.

Oxygen, for example, has the electron configuration of 1s², 2s², 2p⁴ and thus six valence electrons. The first four dots are placed on each side of the oxygen symbol, one dot in each direction.

The remaining two valence electrons are placed on two other sides around the atom. Each side is equivalent and can take a maximum of two electrons.

The number of unpaired dots represents the number of bonds each atom of that element can form.

Elements of the last group, like neon, are noble gases and do not readily participate in chemical bonding. Noble gases have high ionization energies and are very stable given their electron configuration of a full outermost shell.

The Lewis symbol for neon has eight dots, two dots on each side, representing the filled electron configuration; in other words, an octet.

The octet rule states that an atom tends to lose, gain, or share electrons in the form of bonds until a stable electron configuration, an octet, is reached.

Consider carbon dioxide. Carbon has four unpaired electrons and oxygen has two. The number of unpaired electrons represents the number of electrons required to reach an octet. Hence, carbon shares two electrons with one oxygen, and the other two electrons with the other oxygen, so that each atom can achieve an octet.

Hydrogen and helium are an exception to the octet rule since they only have one s-orbital which can hold just 2 electrons; so hydrogen or helium are said to achieve a duet.