Lewis structure

A Lewis structure (also known as a Lewis dot formula, electron dot structure, or Lewis electron dot structure) is a diagram that illustrates how atoms bond within a molecule and where lone pairs of electrons may be located. Gilbert N. Lewis first introduced this concept in his 1916 paper *The Atom and the Molecule*. These structures can be drawn for any molecule held together by covalent bonds, as well as for coordination compounds. They build on the idea of an electron dot diagram by using lines between atoms to indicate shared pairs of electrons in a chemical bond.

In a Lewis structure, each atom is represented by its chemical symbol and placed according to its position in the molecule. Lines (or sometimes pairs of dots) connect bonded atoms. Any leftover electrons that are not shared—known as lone pairs—are shown as pairs of dots next to their respective atoms.

For main group elements from the second period onward, atoms typically react by gaining, losing, or sharing electrons until they achieve a full octet of eight electrons in their valence shell. Hydrogen, however, follows the duplet rule, forming only one bond to complete its valence shell with two electrons.

**Construction and electron counting**

For a neutral molecule, the total number of electrons shown in a Lewis structure equals the sum of the valence electrons from each individual atom—not the maximum possible. Non-valence electrons are omitted because they do not participate in bonding.

Once the total number of valence electrons (t) is known, they are placed using these steps: - First, draw a single line (representing a single bond) between each pair of connected atoms. - Each bond uses two electrons, so if n is the number of single bonds, then t − 2n electrons remain. These are temporarily drawn as dots, one per electron, up to eight per atom (two for hydrogen), minus two for each bond already present. - Distribute these dots first to the outer atoms, then to the inner ones, until no electrons are left. - Any atom (except hydrogen) with fewer than eight electrons (counting each bond as two) is adjusted: for every two electrons needed, remove two dots from a neighboring atom and add an extra line between them. This turns a lone pair into a bonding pair, adding two electrons to the first atom’s valence shell while leaving the second atom’s count unchanged. - If there aren’t enough electrons to fill all valence shells, priority goes to atoms with higher electronegativity.

Polyatomic ions are drawn the same way, but negative ions include extra electrons in the structure, while positive ions have fewer electrons than a neutral molecule. When writing an ion’s Lewis structure, the entire diagram is placed in brackets, with the charge written as a superscript outside the upper right corner.

A simpler method avoids counting electrons: draw atoms with their valence electrons, then form bonds by pairing up valence electrons from the bonding atoms. Anions and cations are created by adding or removing electrons from the appropriate atoms.

Another trick: count the total valence electrons, then count the electrons needed to satisfy the octet rule (or duplet for hydrogen). The difference between these two numbers gives the number of electrons that form bonds; the remaining electrons fill the octets of all other atoms.

A general procedure called the lever method has also been proposed for writing Lewis structures and resonance forms. This system works in nearly all cases, except for three specific exceptions, which are listed in a table.

**Formal charge**

Formal charge is used in Lewis structures to describe, compare, and assess likely topological and resonance forms. It calculates the apparent electronic charge of each atom based on its electron dot structure, assuming purely covalent or non-polar bonding. This helps determine possible electron rearrangements in reaction mechanisms and often matches the sign of the atom’s partial charge, though exceptions exist. The formal charge of an atom is calculated using the formula:

C_f = N_v − U_e − (B_n / 2)

where C_f is the formal charge, N_v is the number of valence electrons in a free atom of the element, U_e is the number of unshared electrons on the atom, and B_n is the number of bonding electrons (shared pairs) around the atom.